Added subscriptions to Phriction documents

Summary:
Fixes T2694

added edge infrastructure for Phriction

added mail subject prefix option for Phriction

added messy mail support for subscribers

adds edges to the phriction db, along with the subscriber interface
which gives us subscriptions for free.

simple display of subscribers, adequate to the current design and
sufficient fallbacks for exceptional cases. @chad may
be mailed about that one more UI element may be added to his redesign

mail support is messy. not generic at all. only sends to subscribed non-authors.

Test Plan:
tried out all kinds of stuff. applied patch, subscribed, unsubscribed with multiple
accs. verified proper

edited documents, verified that mail was sent in MetaMTA. Verified
contents, tos and stuff by looking into the db, comparing PHIDs etc.

functional testing per serious MTA (that is, AWS SES) worked wonderfully.

Here's how the subscription list looks like:
{F36320, layout=link}

Reviewers: epriestley, chad, btrahan

Reviewed By: epriestley

CC: hfcorriez, aran, Korvin

Maniphest Tasks: T2686, T2694

Differential Revision: https://secure.phabricator.com/D5372

Conflicts:

src/infrastructure/storage/patch/PhabricatorBuiltinPatchList.php