`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.
gfyoung