i18n: Get rid of get_lang_from_prefs

The function uses profile.ui_lang which we are not using anymore.
pootle_profile: Get rid of profile.ui_lang
Add jQuery.textcomplete
Add support for auto-completing placeables
Improve placeables inside right-to-left text

Make them embedded left-to-right strings so that things like “%(foo)s”
do not look like “foo)s)%” inside right-to-left strings (which can be
seen when setting, say, Arabic as an alt source language).

This does not affect translation field (which would benefit from this
most), though.
Fix the placement of the auto complete drop down

For some reason we need to force block here otherwise the placement is
off in Firefox.
Escape HTML in autocomplete dropdown
Cleanup: Remove unnecessary checks
Don't migrate buildversion on updatedb to PootleConfig

It won't work because the necessary tables are not yet present.
Also it is not necessary because this upgrade is already performed later
in the upgrade management command.
Recover ability to save buildversion using djblets

Since updatedb cannot rely on having the buildversions in PootleConfig
(see previous commit), we need to get back the legacy code to save the
buildversion using djblets.
Bugfix: Specify removed field instead of relying on model

Since we removed the description_html fields in b44dcf03 (in 2.5.1 and master)
we have to specify the fields instead of relying on the model, because they
do not longer exist there.
Styleguide: Use single quotes for template tags and filters in HTML attributes

So we avoid clashing with the HTML double quotes as much as possible.
Dev docs: Add guidelines on how to use migrations, upgrade and initdb
Cleanup: Provided build versions are always integers

So avoid the conversion.