Accept template contents as strings instead of reading from disk

[JacobsonMT]

What is this feature?

Makes use of the new upstream Template.New() to stop passing user-defined templates to the Grafana Alertmanager by persisting them to disk.

Why do we need this feature?

This PR is a general improvement and simplification, but there are also some specific reasons why we might want to do this:

• Incremental step towards abstracting data layer for Grafana's Alertmanager interface away from needing disk-based storage. Next step would likely focus on FileStore.
• Reduces likelihood of consistency complexities on save/apply, especially in the context of future Remote AM work.
• Since DefaultTemplateString is now treated like other defaults and therefor consistently applied first. This ensures that user-defined templates can now rely on being able to override defaults.

Future improvements

• Similarly, we could consider moving the existing prometheus defaults (default.tmpl and email.tmpl) to strings as well, allowing them to be more easily overridden via API/UI.
• Similar changes can be made in Mimir Alertmanager eventually as well.

[grobinson-grafana]

LGTM! A couple of comments.

[yuri-tceretian]

LGTM.

I do not think that there is anything else to do because we do not let users to override the files on the disc. According to the code https://github.com/grafana/grafana/blob/a564c8c4398fe2a763712fa54e21885017151b45/pkg/services/ngalert/notifier/config.go#L25-L54 we always override the content of the files.

[gotjosh]

LGTM - my comments are nits and I don't need to look at this again.