MDL-47183: viewhiddenactivities should use the module context

Thanks to Paulo Matos for the forum part of this change.
MDL-46215 mod_forum: Improve style of subscription state

With the implementation of per-discussion subscription, the current
discussion's subscription state must be shown.

Since this state belongs to the discussion rather than the forum, it
should be shown accordingly.  This change therefore includes the addition
of the discussion name as a heading on the page.
Merge branch 'MDL-46215-master' of git://github.com/andrewnicols/moodle
MDL-47585 stats: fixed incorrect user dropdown
Merge branch 'MDL-47183-master' of git://github.com/andrewnicols/moodle
MDL-47862 event_monitor: Fix context for capability check on subscription page