Alertmanager: Add default Grafana template string

[santihernandezc]

When using Grafana receivers in Mimir with the default settings some fields in notifications end up empty (e.g. title and message). This PR adds DefaultTemplateString to the *template.Template we use in notifications to fix this issue.

[stevesg]

Whilst we not merging configurations, I'm a little worried about introducing these additional templates into users which just have a standard configuration uploaded. I think I'd prefer them only to be loaded if a Grafana configuration is active.

Why not just add the template into the configuration?

[santihernandezc]

@stevesg I realized that trying to send notifications with upstream's receivers results in errors with the original approach. There are some differences in the structure of alerts between upstream and Grafana.

I created separate *template.Template structs for both sets of receivers, and in my last commit I'm creating the grafana template struct only if necessary.

[stevesg]

Hm, this isn't as simple a problem as it seemed like.... We'll definitely have to re-think the ExternalURL situation when we come to merging the configurations, but this is good enough for now.

Perhaps when we do merge configurations, we'll have to pass two ExternalURLs down to the Alertmanager, one for the standard receivers and one for Grafana ones.