controllers: move controllers base class from lib/base to controllers

TG quickstart put it in lib/base.py , but it fits better on the controllers
layer as a base there.

The contributing docs were a bit ahead of time ... but with a typo.

py3: use html.escape instead of cgi.escape

DeprecationWarning: cgi.escape is deprecated, use html.escape instead

backend: replace Pylons with TurboGears2

Replace the no-longer-supported Pylons application framework by TurboGears2
which is largely compatible/similar to Pylons.
Some interesting history is described at:
https://en.wikipedia.org/wiki/TurboGears

Changes by Dominik Ruf:
- fix sql config in test.ini

Changes by Thomas De Schampheleire:
- set-up of test suite
- tests: 'fix' repo archival test failure
Between Pylons and TurboGears2, there seems to be a small difference in the
headers sent for repository archive files, related to character encoding.
It is assumed that this difference is not important, and that the test
should just align with reality.
- remove need to import helpers/app_globals in lib
TurboGears2 by default expects helpers and app_globals to be available
in lib. For this reason kallithea/lib/__init__.py was originally changed
to include those files. However, this triggered several types of
circular import problems. If module A imported something from lib (e.g.
lib.annotate), and lib.helpers imported (possibly indirectly) module A,
then there was a circular import. Fix this by overruling the relevant
method of tg AppConfig, which is also hinted in the TurboGears2 code.
Hereby, the include of something from lib does not automatically import
helpers, greatly reducing the chances of circular import problems.
- make sure HTTP error '400' uses the custom error pages
TurboGears2 does not by default handle HTTP status code
'400 (Bad Request)' via the custom error page handling, causing a
standard non-styled error page.
- disable transaction manager
Kallithea currently handles its own transactions and does not need the
TurboGears2 transaction manager. However, TurboGears2 tries to enable it
by default and fails, throwing an error during application initialization.
The error itself seemed to be harmless for normal application functioning,
but was nevertheless confusing.
- add backlash as required dependency: backlash is meant as the WebError
replacement in TurboGears2 (originally WebError is part of Pylons). When
debug==true, it provides an interactive debugger in the browser. When
debug==false, backlash is necessary to show backtraces on the console.
- misc fixes

controllers: rename __before__ to _before in preparation of TurboGears2

__before__ in Pylons is called _before in TurboGears2. We can prepare this
rename already in Pylons-based Kallithea, so that the real TG2 migration
commit just changes the BaseController.

Since TurboGears2 _before can pass extra arguments, we add *args and
**kwargs parameters as well.

tg: minimize future diff by some mocking and replacing some pylons imports with tg

No actual tg dependency yet, just a temporary hack faking tg as an alias for
pylons.

Based on work by Alessandro Molina.

error: remove unused Pylons-specific img/style methods

The img and style methods in the error controller are standard Pylons
boilerplate. They are used from the Pylons default error document template.
The purpose of these methods is to serve images and stylesheets from the
Pylons module, rather than having to copy these files into a particular
application installation.

Since Kallithea uses a custom error template with our own styles and images,
these methods are not actually used.

error: fix handling of errors without response

Broken by 6f9dae4d39d9.

error: don't crash on /error/document urls

244c9b103cff was insufficient.

The non-crashing page is however not so relevant ...

error: be prepared for resp without status

I have seen traces that shows that it can happen. Be prepared!

cleanup: avoid some 'except Exception' catching - catch specific exceptions or log it and show what happened

This has a risk of introducing regressions ... but we want to get rid of all
exception muting and make the whole system less fragile and easier to debug.

error: remove allmost unused code for redirect on error page

It is much better to just stop and show the error message.

Correct licensing information in individual files.

The top-level license file is now LICENSE.md.

Also, in various places where there should have been joint copyright holders
listed, a single copyright holder was listed. It does not appear easy to add
a link to a large list of copyright holders in these places, so it simply
refers to the fact that various authors hold copyright.

In future, if an easy method is discovered to link to a list from those
places, we should do so.

Finally, text is added to LICENSE.md to point to where the full list of
copyright holders is, and that Kallithea as a whole is GPLv3'd.

Second step in two-part process to rename directories.
This is the actual directory rename.