Always update frontend fixtures (generate if necessary)

There are two ways to run Karma tests:

• bundle exec rake karma which always runs bundle exec rake karma:fixtures and regenerates all frontend fixtures
• yarn karma which never runs bundle exec rake karma:fixtures and therefore assumes the fixtures are already there/updated

We should introduce a way to always run the Karma tests with updated frontend fixtures but without regenerating all of them.

Plan

• store the generated fixtures in the repository
• add a static analysis script to make sure fixtures are updated
• document that fixtures no longer need to be generated unless HAML templates or JSON responses change (https://docs.gitlab.com/ee/development/testing_guide/frontend_testing.html#karma-test-suite)