Add *Collection classes

In their current state, they should help in efficiently fetching certain
revisions of a certain post/header.

I would like to see them be "expanded" to hold stuff that we currently do in
revisions, but is not really revision-specific (e.g. getChildren, getCreator...)

I'll use this to change/fix some permissions in a follow-up patch, which should
make it easier to load previous & most recent revisions.

Change-Id: If53f0b4a1367f4cd7087dc0fad3326436f3a6d9f
Merge "Add *Collection classes"