cleanup: minor refactorings and simplification of dict usage

Makes it more py3 compatible.

pygments: reimplement get_lem under the name get_extension_descriptions

The old implementation was cryptic and over-engineered. And reduce() is not a
builtin in Python 3.

This function works on static input, and it was verified that this
implementation returns exactly the same as the old one. And is simpler and
slightly more readable. In my opinion.

Inspired by change by Lars Kruse.

config: remove unused symbols DATE_FORMAT / DATETIME_FORMAT

These were previously imported by helpers.py, but without actual usage
anywhere.

pygmentsutils: separate from util2

__get_lem and __get_index_filenames has nothing in common with the rest
of util2, neither with respects to implementation nor usage, and belongs
in a dedicated module. Also, they are clearly not actually private, so
shouldn't be named with leading underscores.

indexers: introduce list of filenames for indexing

Before this patch, we cannot search files that without file extension (like
Makefile) and dotfiles (like .hgtags).

This patch makes it possible to search for these files by introducing a list
of filenames for indexing. The list is currently empty.

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.