R1:d5841fe4991c

Added basic hovercard controller

Summary:
Refs T1048; Depends on D5542, D5543, D5544 - It currently just renders multiple hovercards nicely for test purposes. More is on the way.

Mode `test`: Human test chamber.
Mode `retrieve`: For JS. Added so it would not clash with search key routing.

badassery

Test Plan:
`/search/hovercard/test/?phids[hover-T4]=PHID-TASK-g5pduvwrrwvkq5gkx736&phids[hover-T2]=PHID-TASK-gta6lzaaagziavkktima`

Verified the appearance of two tasks with correct rendering and correct ids…