Fix some i18n issues and use djblets localization.

This change makes a bunch of small improvements to localization in Beview
Board:
* Switch from ugettext to ugettext_lazy in places where _() is called in a
  "static" way.
* Add 'djblets' to INSTALLED_APPS, so that djblets/locale will be added to the
  locale paths.
* Add 'djblets' to the list of packages which are included in the jsi18n view.
* Switch admin/checks.py to use 'ugettext' instead of 'gettext', since the
  latter just crashes if it sees unicode.

Testing done:
Checked that the djblets i18n worked properly for both backend python/template
strings, as well as front-end javascript strings.

Reviewed at https://reviews.reviewboard.org/r/5091/