Complete extraction of all observers to registries

Now that a requirement has arisen to modify the manual observers ([see trello ticket](https://trello.com/c/lNtm0h4x/251-manuals-appear-in-site-search-add-an-observer-to-the-manual-publication-flow-which-exports-stuff-to-rummager-2)), 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.
We didn't previously -- I'd intended for this to be a no-op change from the POV of functionality. See https://github.com/alphagov/specialist-publisher/pull/202/files#diff-8fa796acfacc8370b956c1a0ff58fc75L17