[IDEA] Fix code templates

[mgaffigan]

Code templates are implemented weirdly and cause a bunch of odd behavior: the template library controls to what channels they apply and what templates they own. That's a bit backwards.

Desired:

• Normalize the database schema to have code templates FK out to code template libraries
• Implement standard CRUD for Code Template Libraries and Code Templates (avoid persisting lists - see [BUG] Importing a channel can delete recently added code templates #296)
• Add dependency list for channels into the channel itself, not separate from the channel
• Add migrations for all of the above, considering code template and channel imports