This reverts merge request !29733

Use NotesFinder to fetch notes in module IssuableActions

Closes #52123

See merge request gitlab-org/gitlab-ce!29733

Add spec for concern IssuableActions

Add shared samples for discussions endpoint

Add schema validations for discussions

Fix rubocop style issue

Make target assignable

Use new possibility to provide target

These are not required because MySQL is not
supported anymore

We use a leading dash for certain things like the WebIDE, which
had the side effect of losing the `params[:continue][:to]` param when
opening the WebIDE on a project where the user doesn't have push access
and therefore needs to fork the project first.

The check for continue_params&.key?(:to) in Projects::ImportsController
caused an exception in redirect_to if this key contained a nil value.

Since url_for won't add any params for an empty hash, we can just return
that in continue_params if params[:continue] isn't present, and simplify
the code in the controllers to check for the values we actually want to
use.

This fixes a regression in mime-types-data:
https://github.com/mime-types/mime-types-data/pull/20