Generalize a couple implementations of EchoGetDefaultNotifiedUsers

There are a variety of generic strategies you could define to choose
which users should be notified about an event, such as 'users watching
the title' or 'talk page owner' (User_talk only).

This adds a new way to generically implement these in Echo and expose them
to other extensions rather than having each extension implement these
generic strategies themselves.

This patch only converts one notification, edit-user-talk. The remaining
notifications will be converted in future patches. The first user of this
will be Flow for notifying all users watching a particular talk page in
I4e46a9c003fbdde274b20ac7aef8455eab4a5222

The users watching title implementation provided here is minimalist, a larger
refactor to accomidate pages with thousands of watchers is being handled
in I3d3fa9328f348bb48682d3658622952ce82d3925

Change-Id: I19bb6a794d22565f3bb5421de92426d390197796
Merge "Generalize a couple implementations of EchoGetDefaultNotifiedUsers"