Resolve "Merge approvals don't appear to trigger notification emails"
This adds:
- An 'approval required' todo type (I'll need to create an MR to CE with this commit after this MR is done).
- A todo for approvers on MR creation. (They already get an email, as they are participants in the MR.)
- A todo and an email for new approvers when an MR is edited.
Closes #42 (closed).