bug 1401246: Replace dbgettext with management command

[jwhitlock]

django-dbgettext was used to extract the Filter and FilterGroup names from the database and create files so that they would be picked up as localizable strings. There's a few issues:

• This process wasn't documented. We haven't updated the strings for changes to the filters, so they are badly out of sync (see French topic filters).
• The output is gross, spread over lots of files in kuma/core/translations.
• We don't have a method for regenerating the files against the production database and then checking in the changes, so any updates would have to be done manually.
• The project is barely maintained, with a minimal release in 2016 for Django 1.9 compatibility. It uses an auto-discover process that hasn't been updated for AppConfig-style apps, added in Django 1.7.
• I hope to eventually move Filter and FilterGroup out of the database and into code. I'd like to avoid UI strings defined in the database in the future (I'm looking at you, DocumentTag).

This PR:

• Drops django-dbgettext, output files, and configuration.
• Replaces it with a Django management command generate_search_names.
• Adds kuma/search/names.py with the current names from production.
• Adds NAME: "jinja2" to the TEMPLATES setting, so that I can get the Jinja2 engine by that name rather than the default backend.

[escattone]

Thanks @jwhitlock!