Signout URL handling improvements

For https://repo1.dso.mil/platform-one/big-bang/apps/core/monitoring/-/issues/15

BB MR here - https://repo1.dso.mil/platform-one/big-bang/bigbang/-/merge_requests/1879 - must be used to prevent overriding from the BB level.

Tested with both sso enabled and disabled, in both cases the signout is handled as expected, with Grafana redirecting the user to the login page and remove their authorization.