Convert training_progress to generic TemplateView

[elichad]

As Get Involved form and related processing has been removed from the training_progress view:

Given how it has been simplified, it should be turned into a generic TemplateView. Can be done later as long as there's a ticket for it.

Originally posted by @pbanaszkiewicz in https://github.com/carpentries/amy/pull/2492#discussion_r1283720996