Add check to config that notifications have the right tags

[philayres]

Notifications triggered on the creation or save of a dynamic definition can substitute data using {{tag}} substitutions. The problem is that a notification that contains tags to be substituted will cause an exception in a background job if the "source" data doesn't contain all the tags (to prevent incomplete messages being sent to users).

Ideally, configurations should check referenced notifcation templates (the the directly defined template text) to ensure that all tags are potentiatlly available in the data. This shouldn't actually instantiate any data, but instead ensure that any referenced data in the current model (or for tags with {{associated_model.data}} in the associated model) actually appear as attributes or one of the valid extra attributes added by the substitution process.

It may be that this is done by a new message template validator, which could delegate certain checks to the tag substitutor.