Try to simplify feature flag checks by using policies

Enables frozen string for the following:

* app/controllers/*.rb
* app/controllers/admin/**/*.rb
* app/controllers/boards/**/*.rb
* app/controllers/ci/**/*.rb
* app/controllers/concerns/**/*.rb

Partially addresses #47424.

When the database is in a read-only state, display a banner on each
page informing the user they cannot write to that GitLab instance.

Closes gitlab-org/gitlab-ce#43937.

In GitLab EE, a GitLab instance can be read-only (e.g. when it's a Geo
secondary node). But in GitLab CE it also might be useful to have the
"read-only" idea around. So port it back to GitLab CE.

Also having the principle of read-only in GitLab CE would hopefully
lead to less errors introduced, doing write operations when there
aren't allowed for read-only calls.

Closes gitlab-org/gitlab-ce#37534.

Modifies the existing "login as" feature to be called impersonation, as
well as keeping track of who is impersonating to revert back to that
user without having to log out.

Signed-off-by: Jeroen van Baarsen <jeroenvanbaarsen@gmail.com>

Handles stuff that's shared across admin controllers.