vcs: drop GitChangeset.id

It seems to be like raw_id, except the odd feature that it sometimes would
return 'tip'.

lib: de-obfuscate nested functions in pygmentize_annotation and how they always pass an annotate_from_changeset_func to AnnotateHtmlFormatter

These functions contain the only call to annotate_highlight, which contains
the only call to AnnotateHtmlFormatter.

annotate_highlight is thus clearly always passed an
annotate_from_changeset_func.

vcs: always return bytes from node.content

We will rather have the unicode conversions explicit.

Note: Py3 bytes doesn't have .startswith - replace that with a regexp.

codingstyle: trivial whitespace fixes

Reported by flake8.

lib: move get_custom_lexer from utils to pygmentsutils

get_custom_lexer is the only dependency from helpers to utils.
In attempting to get a clearer dependency tree, we can move out
get_custom_lexer to a different place so that helpers does not depend on
utils.

It so happens that there already is a pygmentsutils.py file in lib, which is
a very good fit, since the lexers used in Kallithea are effectively provided
by pygments.

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.