fix incoming GTS mentions and DMs

GoToSocial sends 'tag' values as a single object if there is only one
user mentioned, rather than an array with an object inside it.

This causes Bookwyrm to reject the tag since it comes through as a
dict rather than a list.

This commit fixes this at the point the incoming AP object is transformed
so that "mention" tags are turned into a mention_user.

bookwyrm/models/fields.py

@@ -379,6 +379,11 @@ class TagField(ManyToManyField):
 
     def field_from_activity(self, value, allow_external_connections=True):
         if not isinstance(value, list):
+            # GoToSocial DMs and single-user mentions are
+            # sent as objects, not as an array of objects
+            if isinstance(value, dict):
+                value = [value]
+            else: 
                 return None
         items = []
         for link_json in value:

bookwyrm/models/status.py

@@ -136,10 +136,9 @@ class Status(OrderedCollectionPageMixin, BookWyrmModel):
         # keep notes if they mention local users
         if activity.tag == MISSING or activity.tag is None:
             return True
-
-        # BUG: this fixes the TypeError but if there is only one user mentioned
-        # we still don't get any notifs and DMs are dropped.
-        tags = activity.tag if type(activity.tag) == list else [ activity.tag ]
+        # GoToSocial sends single tags as objects
+        # not wrapped in a list
+        tags = activity.tag if isinstance(activity.tag, list) else [ activity.tag ]
         user_model = apps.get_model("bookwyrm.User", require_ready=True)
         for tag in tags:
             if (