Merge branch 'fix/activity_get_by_id' into 'develop'

Add get_by_id to activity.ex

See merge request pleroma/pleroma!695

lib/pleroma/activity.ex

@@ -36,6 +36,10 @@ def get_by_ap_id(ap_id) do
     )
   end
 
+  def get_by_id(id) do
+    Repo.get(Activity, id)
+  end
+
   # TODO:
   # Go through these and fix them everywhere.
   # Wrong name, only returns create activities