Complete extraction of all observers to registries

Now that a requirement has arisen to modify the manual observers, it's
time to finish off nuking the observers entirely from the wiring.

Extract Manual, Manual Document and Aaib Report observers into their own
registries and nuke the original 'catch-all' observers.
Merge pull request #202 from alphagov/extract-all-observers-to-registries

Complete extraction of all observers to registries