Add documentation on Xapian and virtualenv's

Fix bug 2972
Add link to installation instructions for other platforms
Disable directory indexes by default

Turn off directory listing by default
Add back listing for exports/

Fix bug 2936
Python 2.5 deprecation support

In addition to Jinja, latest release of South has dropped 2.5 support.
The list will clearly continue to grow longer - so now it has its own
file.
Fix sub-list reStructuredText formatting

When using sub-lists, blank lines are required between list items;
if omitted, sub-lists are treated as normal flowed paragraphs with
embedded 1. or * in the text.

(Blank lines between list items are always allowed, and are usually
a good idea, but are optional if there are no sub-lists or embedded
un-numbered or -bulleted paragraphs between the list items.)
Make the sub-lists Markdown-compatible

This isn't strictly necessary, since these are reStructuredText docs, but
you never know when somebody might cut & paste your useful advice for use
in another context, where Markdown formatting is more likely.

For Markdown to recognize a sub-list, it has to be indented by 4 to 7
spaces.  Just indenting the sub-list items would work for Markdown, but in
reStructuredText, all the main list items will become bold and you will be
*very* confused.  (Indenting the sub-list items more than the main list text
makes them look like definition lists, where the "defined word" is bolded.)
So you need to indent the sub-list items by 4, and then add extra space
between the bullet or number and the text in the main list items that have
sub-lists, so that the main list text is aligned with the sub-list numbers
or bullets.  Now everybody is happy.

And you thought these plain-text formatting/markup languages were simple?
Improve punctuation

Put ? at end of questions (even rhetorical ones), italicize i.e. and e.g.,
Some minor rewording (use `git diff --color-words='[[:alnum:]_-]'` to see it)
Offer XLIFF download links

Fix bug 2966
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
API: Use default Language manager for LanguageResource

This way it is possible to access to all languages and not just those that
are used on at least a translation project.