Better handling of bases without translations

[rolandgeider]

If a base has no translations many things can break (#1269, wger-project/flutter#359).

While ideally this shouldn't happen, we could handle this better:

• In the exercisebaseinfo only iterate over the bases with translations
• Add a check that sees if the exercises have problems (error when no translations and warning when no English ones?), see core/checks.py
• Make the submission process more robust so that this can't happen
• Make sure the search also doesn't break

[rolandgeider]

started adding custom managers that automatically filter out exercises without translations. Still need to test this a bit and see if there are some performance problems