Add a blacklist editor that saves to e621

The Online sidebar's "Your blacklist" section is editable now: typing a
tag appends it and each entry gets an x to remove it. Changes are written
straight to the e621 account (PATCH /users/{id}.json with
user[blacklisted_tags]); the store keeps the fetched profile for the id,
updates the list optimistically and reloads it from e621. The Followed
page's blacklist cloud is rebuilt afterwards via the new
/api/follows/cloud/?refresh=1 force flag.

Verified against the live API with a reversible add/verify/restore test.

backend/apps/follows/views.py

@@ -282,10 +282,13 @@ class FollowCloudView(APIView):
     permission_classes = [IsAuthenticated]
 
     def get(self, request):
+        force = request.query_params.get("refresh") == "1"
         cloud, _ = FollowCloud.objects.get_or_create(user=request.user)
         now = timezone.now()
-        stale = cloud.computed_at is None or now - cloud.computed_at > (
-            cloud_service.CLOUD_TTL
+        stale = (
+            force
+            or cloud.computed_at is None
+            or now - cloud.computed_at > (cloud_service.CLOUD_TTL)
         )
         if stale:
             reclaim = Q(building=False) | Q(