Bundle custom.css into css_common bundle

custom.css now part of css_common bundle. Updated notes in
customizations.rst and changelog.rst

Fix bug 2934
Make lists more readable
Put rebuild instructions in big red warning
Use :wiki: and :wp: markers
Rather point to the PyPI version of Levenshtein

This is after all what is being installed via pip.  It also means that
we can drop our own version on sourceforge in future.
Point to upgrading :doc: not :ref:
Fix wording
Don't use headings for the link text

Mostly they don't read well in the context of the sentence and have
wrong capitalization
Add mising select2 image
Correct select2 reference hash

Checked with Julen on this.  Seems the has got changed when upstreamed.
Actually commit the png
Fix URLs
consistent spelling