- concerns
- dashboard
- groups
- import

To prevent an OAuth2 covert redirect vulnerability, this commit adds and
uses an alias for the GitHub and BitBucket OAuth2 callback URLs to the
following paths:

GitHub: /users/auth/-/import/github
Bitbucket: /users/auth/-/import/bitbucket

This allows admins to put a more restrictive callback URL in the OAuth2
configuration settings. Instead of https://example.com, admins can now use:

https://example.com/users/auth

It's possible but not trivial to change Devise and OmniAuth to use a
different prefix for callback URLs instead of /users/auth. For now,
aliasing the import URLs under the /users/auth namespace should suffice.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/56663

Enables frozen string for the following:

* app/controllers/dashboard/**/*.rb
* app/controllers/explore/**/*.rb
* app/controllers/google_api/**/*.rb
* app/controllers/groups/**/*.rb
* app/controllers/import/**/*.rb
* app/controllers/instance_statistics/**/*.rb
* app/controllers/ldap/**/*.rb
* app/controllers/oauth/**/*.rb
* app/controllers/profiles/**/*.rb

Partially addresses #47424.

This whitelists all existing offenses for the various CodeReuse cops, of
which most are triggered by the CodeReuse/ActiveRecord cop.

Importing a project from GitHub for a project namespace that already exists
would show an unhelpful error, "An error occurred while importing project."
We now add the base message from Projects::CreateService when this fails.

Closes #47365

Closes #29739