Integrated Alerting: Entities (e.g. templates, channels, rules) are in inconsistent states

Keep IA entities (templates, channels, rules) at a consistent state.

Prevent channel deletion if it's used by some rules.

Skip alerts in ListAlerts response where related rule not found
Skip alerts in ListAlerts response where related template not found

How to test

Case 1:

1. Create notification channel
2. Create rule
3. Try to remove created channel (error expected)

Case 2:

1. Create rule
2. Wait until an alert appears on Alerts page
3. Remove created rule
4. Open Alerts page (no alert, no errors)

Case 3:

1. Create template file in /srv/ia/templates  (it could have some fake expression like "10 > 9")
2. Open Alert Rule Templates page to trigger templates reloading
3. Create Rule
4. Wait until an alert appears on Alerts page
5. Remove created template file
6. Open Alert Rule Templates page to trigger templates reloading
7. Open Alerts page (alert should disappear, no errors)