Merge branch '42-merge-approvals-don-t-appear-to-trigger-notification-emails' into 'master'

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.

See merge request !547