Add check to prevent multiple follow notifications from the same user

Fixes #397 (closed)

Previously, a follow->unfollow->follow chain potentially caused by spam-clicking would raise multiple notifications on the followed user's account

This adds a check on activity type = "Follow", and will not insert a notification if a duplicate already exists in the user's last 20 notifications

This would not affect longer-timespan chains, and only prevent short-term duplicates