Audit what we put in initializers and consider moving some of them in a more correct place

Everyone can contribute. Help move this issue forward while earning points, leveling up and collecting rewards.

• Close this issue

Currently we tend to throw a lot of stuff in initializers:

• Monkey patches: https://gitlab.com/gitlab-org/gitlab-ee/blob/master/config/initializers/0_as_concern.rb
• Rails environment hacking: https://gitlab.com/gitlab-org/gitlab-ee/blob/master/config/initializers/0_post_deployment_migrations.rb
• Config validation: https://gitlab.com/gitlab-org/gitlab-ee/blob/master/config/initializers/6_validations.rb
• Set Rails environment values: https://gitlab.com/gitlab-org/gitlab-ee/blob/master/config/initializers/default_url_options.rb
• Plug rack middleware: https://gitlab.com/gitlab-org/gitlab-ee/blob/master/config/initializers/etag_caching.rb
• Show message to the end-user using view helpers: https://gitlab.com/gitlab-org/gitlab-ee/blob/master/config/initializers/license.rb (see https://gitlab.com/gitlab-org/gitlab-ee/commit/8177803eec3956625eaf2a6def420d4540fcc3de#note_25058229)
• Do weird stuff: https://gitlab.com/gitlab-org/gitlab-ee/blob/master/config/initializers/math_lexer.rb

I think we should audit what we do here and act consequently (plan to be defined).