Support Mute Timings within Unified Alerting

[gotjosh]

Support for recurring silencing is a hot topic for alerting within Grafana per #36637 and the most voted feature under the alerting label per #6592.

The Alertmanager upstream has recently implemented a feature that supports this prometheus/alertmanager#2393 which means we can easily implement it within Grafana too.

A high-level list of steps to take are:

• Mute Timings within Unified Alerting - UX work #40291
• Mute timings - UI #41285
• Support CRUD of <mute_time_intervals> through the API - doc link
• Support attaching <mute_time_intervals> to a particular route. doc link
• Pass the <mute_time_intervals> configuration to the Alertmanager.
• Update the documentation
• Close the two issues mentioned above.

[armandgrillet]

I've talked with the UX team about this, the plan is to start working on the UX the second week of September and then have the front-end and back-end work starting. This means missing the 8.2 release and being done around the end of September.

[armandgrillet]

Removed @kylebrandt from the assignees as he is moving back to the Grafana team but this will need backend work.

[armandgrillet]

@nathanrodman Have you talked to Jessica about this? AFAIK we need her opinion before moving forward with front-end/UX work.

[nathanrodman]

@armandgrillet I've been meeting with Jess on this and we have some initial UX designs that we're working through

[armandgrillet]

Initial UX work: https://www.figma.com/file/2s7EGevGeI4GuiArOmUECK/Unified-Alerting?node-id=2051%3A7907