extensions: rename 'rcextensions' into 'extensions' but provide compatibility

The 'rc' prefix is legacy.
Rename 'rcextensions' into 'extensions', updating all references.
Compatibility with the old name will be retained for a while, and removed in
a later release. Migrating is as simple as renaming a file.

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.

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.

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.