Introduce ember-changeset and ember-changeset-validations

# Problem

Too much complexity is wrapped up in `save` right now when dealing with models.

Where possible, we should work with changesets, using [`ember-changeset`](https://github.com/DockYard/ember-changeset), which will also allow us to use validations.
## Subtasks
- [ ] Identify places where we can introduce changesets
- [ ] Break out separate issues for tackling these

Start by locating the model save flows where complexity is currently concentrated, then identify which places could use changesets and validations. The work is complete when those candidate areas are documented and separate issues have been created for each concrete change.

Written by the indexing model from the issue text.