Alerting: Simplify contact point template selection

[gillesdemey]

Problem

Currently selecting a template for either the body / subject of a contact point is rather error-prone.

As can be seen in the screenshot below you have to

1. use the correct Go template syntax
2. use the correct name of the template defined in the notification template – and these may not be the same (which is confusing).

Potential solution

Ideally we'd allow for the user to make a selection from a list of templates (from a combination of notification templates – keep in mind that a "notification template" can embed multiple "templates" – yes, this is confusing).

Approach

We can approach this in two phases;

• Allow selection from a list of templates
• Preview the selected template contents (optionally a rendered preview but TBD how that would work in the context of a contact point)

Important note

Let's keep in mind that – for any reason – the user might still want to manually fill in a value here, for example when a fixed value or inline template needs to be defined.