Refactor "templates.ts"

## Description
`templates.ts` has grown to over 1600 lines of code.
This could be a good opportunity to refactor it to a library folder (see lib/invitations) with one-function per file to make code review, testing, and modification easier.

The function files could then further be refactored to break particularly long functions up into supporting _private functions.
Tests and imports across the project would need to be updated of course.

`templates.ts` is not unique in this regard, the are likely other lib files that could be similarly refactored in future.