Revise translation pipeline

[bruvellu]

The previous translation pipeline was all done locally. I would run:

cd meta
../manage.py makemessages --no-obsolete --all
../manage.py compilemessages

Then go to our web translation interface http://127.0.0.1:8000/rosetta/ and edit the translations, which would get automatically saved to the MO and PO files:

meta/locale/en/LC_MESSAGES/django.mo
meta/locale/en/LC_MESSAGES/django.po

I would commit the changes to the local copy and push it to the repository and production. This worked well for a single editor.

However, now that the database will be continuously updated by other users, including the translations, it is time to re-think the translation pipeline.

[bruvellu]

Revised the translation pipeline in several commits ending in ef431720bfe90453031aee57de95bdef0f910486.

Also wrote the documentation for dealing with translations at docs/translations.md