Split up creating, deleting and updating diag requests for clarity.

Now reads the 'action' field of an incoming diagnostic request.
Don't allow updating recurring diag requests (KISS).

Just delete it and re-add. For a proper update we would need a way to
tell exactly which fields you want to update, and from there things get
ugly.
Return a status response for all diagnostic request commands.
Add tests for adding non-recurring diag requests and cancelling.
Merge remote-tracking branch 'origin/action-field-diag-request' into action-field-diag-request

Conflicts:
	src/commands.cpp
Merge branch 'action-field-diag-request' into next