Add content-type endpoints for updating templates and template properties

This MR adds content-type endpoints for updating templates and template properties. It ended up being way bigger than I initially anticipated. I could try splitting it into smaller MRs, but I do not think that it would make much sense in most cases. The easiest way is probably to review each commit individually. However, there were some design decisions that were reverted in later commits (for example, LooseTemplateProperty classes).