This commit adds an API to `zproject/urls.py` to edit/update
the realm linkifier. Its helper function to update the
database is added in `zerver/lib/actions.py`.
`zulip.yaml` is documented accordingly as well, clearly
stating that this API updates one linkifier at a time.
The tests are added for the API and helper function which
updates the realm linkifier.
Fixes#10830.