Add basic "Subscriptions" application
Summary:
Basic infrastructure for generalizing subscriptions/CCs for T1808, T1514 and T1663.
- Implement PhabricatorSubscribableInterface and you'll get a subscribe/unsubscribe button for free.
- If there are any auto-subscribed users (like the question author) you can specify them; this makes more sense for Tasks and Revisions than Ponder probably, but maybe the author should be auto-subscribed.
- Subscriptions are either "explicit" (the user clicked 'subscribe') or "implicit" (the user did something which causes them to become subscribed naturally). If a user unsubscribes, they'll no longer be added by implicit subscriptions. This may or may not be relevant to Ponder but is an existing Herald feature in Differential.
- Helper method on PhabricatorSubscribersQuery to load subscribers.
- This doesn't handle actually sending email, etc. I think that's all so application-specific that it doesn't belong here.
- Now seems to work.
Test Plan:
{F20552}
{F20553}
Reviewers: pieter, btrahan
Reviewed By: pieter
CC: aran
Maniphest Tasks: T1663, T1514, T1808
Differential Revision: https://secure.phabricator.com/D3637
