cleanup: remove unused imports

Found with pyflakes.

cleanup: make module self-naming consistent

The self naming is quite pointless ... but at least make it right.

hg: drop support for hg web.hidden

Kallithea is not hgweb and should not try to support hgweb config options.

If someone wants to hide repos, they should just remove all access to them.

(_If_ Kallithea should support some kind of more thorough repo hiding, it
should be implemented differently: The hidden repos should not be in the
database and should thus instead be excluded when scanning for repos.)

urls: introduce canonical_url config setting

All URLs that are shown or persisted or emailed will use this instead of the
current url.

This is convenient when the server has multiple names - for instance when
transitioning from one protocol or domain or hostname to another.

SETTINGS_PREFIX for identifiers (e.g., db table names) incl. project's name.

kallithea.SETTINGS_PREFIX is a variable string used as a prefix for specific
external identifiers, such as database table names (and likely later form
fields), so that the name of the project need not necessarily be encoded into
data.

This setting is configurable so that compatibility with old, similar
databases can be maintained at the users' request.

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.