Add API method to remove a suggestion (#7978)

DELETE /api/v1/suggestions/:account_id

When blocking, remove suggestion from both sides. Muting not affected,
since muting is supposed to be invisible to the target.

app/models/concerns/account_interactions.rb

@@ -203,7 +203,8 @@ module AccountInteractions
 
   private
 
-  def remove_potential_friendship(other_account)
+  def remove_potential_friendship(other_account, mutual = false)
     PotentialFriendshipTracker.remove(id, other_account.id)
+    PotentialFriendshipTracker.remove(other_account.id, id) if mutual
   end
 end