Integrated Alerting: Ability to delete alert rule templates

User Story:

• As a PMM admin user, I need to be able to delete some AT, so they will not be offed for new Alert Criteria creation, since I don't need them more.  

UI: 

• Delete button/action for AT row 
• Confirmation modal window, before deletion 

Acceptance criteria:

• User asked confirmation before Alert Template is deleted  (standard confirmation box, see Grafana  UI )
• Deleted AT is not possible to be selected during the Creation of Alert Criteria 
• Deletion of AT with this Template been used should show standard error message 
• Delete action should be disabled for built-in templates