Handle internationalization in djblets.

This change does a bunch of stuff to get djblets ready for i18n. The various
pieces of this are:
* Marking all strings for translation (most were already done).
* Switching code that was using ugettext in a "static" context to use
  ugettext_lazy instead.
* Rearranging a little bit of the javascript to format localized strings into
  underscore templates along with the data, rather than encoding text in the
  template itself.

Testing done:
1. Generated the .po files and copied over some data from matching strings
   into the zh_TW .po one.
2. Compiled the .po files to .mo
.. In conjunction with some Review Board changes to include the djblets locale
   paths, loaded various things that used the djblets localization. Saw that
   text in both back-end python/template and front-end javascript code was
   translated.

Reviewed at https://reviews.reviewboard.org/r/5090/