R1:489f9e7dfee8

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…