appview: remove `Rkey` from `models.Follow`
From model layer, we expect follows to be unique per (did, subject). But
from PDS, there is no such constraint. So hide the rkey from model while
keeping duplicated records in DB.
We keep duplicated records in DB to delete them all at once on unfollow
Signed-off-by: Seongmin Lee <git@boltless.me>