Split exercise translations (resolves issue #448)

[nikithamurikinati]

Proposed Changes

• resolves issue Split exercises from translations #448
• adds ExerciseBase class to contain language common fields (category, muscles, muscles secondary, equipment)
• updates all exercise mentions to this new format

Please check that the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• New python code has been linted with with flake8 (flake8 --config .github/linters/.flake8)
  and isort (isort)
• Added yourself to AUTHORS.rst

Other questions

• Does this PR introduce a breaking change such as a database migration? (i.e.
  what changes might users need to make in their running application due to
  this PR?)
  yes migrations must be rerun to update the exercises in the database to fit the new format

[rolandgeider]

This is a big one, congratulations!

Here are a couple of small things:

• I'm getting an

  AssertionError at /api/v2/exerciseinfo/
  The field 'exercise_base' was declared on serializer ExerciseInfoSerializer, but has not been included in the 'fields' option.
  

  error when visiting the new exercise info endpoint.

• Since we have a new Exercisebase, could you add "exercises.exercisebase" to the filter-fixtures file? (the line that mentions exercises.json)

[rolandgeider]

Just merged this, thanks again!