mgr/dashboard: fix errors related to frontend service subscriptions.

mgr/dashboard: fix errors related to frontend service subscriptions.

- Created TimerService: for getting a scheduled observable that checks if there are any observers subscribed before sending a request.
Now FeatureTogglesService uses it internally (solving a hidden bug in it).
- SummaryService & RbdMirroringService: instead of having window.setInterval/setTimeout tasks launched in constructor, now they have a method for polling (and subscribing to it). These methods are called in the appropriate component so we unsubscribe properly.
- Fixed: some subscriptions were not being unsubscribed properly.
- RbdFormComponent: little refactoring and fixed (also improved) unit tests.

Fixes: https://tracker.ceph.com/issues/44228
Signed-off-by: Alfonso Martínez <almartin@redhat.com>