Flow: Add Mention notification.

Currently requires a double-parsing of wikitext to HTML because we
no longer use Parsoid to convert to HTML.

Also includes a refactoring of the notification code, attempting to
put it in its own self contained module. Unfortunately, this means
that it needs to be passed all the way down the tree from
WorkflowLoader in order to be used.

Now also includes a general improvement of notification code, such
as the addition of primary and secondary links, emailing and
bundling.

Dependent on talk page occupation because I've pre-adjusted it to
be compatible with my refactoring of UrlGenerator

Change-Id: Ic6bcd10cfd2cb96a5e0b13f814628fcc6df5d9ef
Merge "Flow: Add Mention notification."