validators: introduce InvalidCloneUriException instead of throwing bare Exceptions for invalid clone URIs

When adding a new repository with a remote clone URI, the URI will be validated
in some way. In case of errors, it would raise 'Exception' to report invalid
URLs. Code calling the validation would thus have to catch 'Exception', which
means that _any_ exception would cause the URI to be found invalid.

Instead, create a special exception type intended to be used for all exceptions
we know can occur during normal URL validation.

locking: drop the pull-to-lock / push-to-unlock functionality

The feature is not worth the maintenance cost. The locking is too coarse and
unflexible with insufficient UI and UX. The implementation is also quite
invasive in tricky areas of the code, and thus high maintenance.
Dropping this will enable other cleanup ... or at least make it easier.

model: changeset_status: remove support for 'dont_allow_on_closed_pull_request' and 'StatusChangeOnClosedPullRequestError'

Previous commit removed the only usage of this parameter and exception.

lock: fix encoding error when Mercurial emits a localized warning with the Kallithea "repository locked" message

Mercurial use (utf8) encoded localized messages. Things failed when it tried to
convert the encoded Mercurial format string to unicode so it could insert
Kallithea's unicode (but ascii compatible) "repository locked" exception
string.

That could for example be reproduced by running the (manual) vcs test
test_push_on_locked_repo_by_other_user_hg with environment LANG=de_DE for the
server side.

To fix this, make sure the "repository locked" message is encoded so Mercurial
handles it correctly.

pullrequests: pass around reviewer User objects, not IDs

This moves reviewer user validation into the controller and eliminates
the UserInvalidException.

pullrequests: detect invalid reviewers and raise HTTPBadRequest

Normally, the creation of a pullrequest with invalid reviewers is not
possible because the list of reviewers is populated from a form element
that only shows valid reviewers.
However, if creating a pullrequest through an API call, invalid reviewers
can be specified but would not be detected. The reviewer would be encoded
in the database as 'NULL'/None, and opening such a pull request would cause
a server error.

Instead, detect invalid reviewers at pullrequest creation/update time and
raise HTTPBadRequest.

admin: users: factorize check for default user

Note that one specific unittest has been commented because it relies on
pytest features (monkeypatch). When pytest is the default test runner, the
test should be uncommented.

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.