sessions_controller.rb

`InternalRedirect` prevents Open redirect issues by only allowing
redirection to paths on the same host.

It cleans up any unwanted strings from the path that could point to
another host (fe. //about.gitlab.com/hello). While preserving the
querystring and fragment of the uri.

It is already used by:

- `TermsController`
- `ContinueParams`
  - `ImportsController`
  - `ForksController`
- `SessionsController`: Only for verifying the host in CE. EE allows
   redirecting to a different instance using Geo.