Added .imageonlychannel / .imageonly to prevent users from posting anything but images in the channel

src/NadekoBot/data/strings/commands/commands.en-US.yml

@@ -2114,3 +2114,9 @@ nhentai:
   args:
     - "273426"
     - "cute girl"
+imageonlychannel:
+  desc: |- 
+    Toggles whether the channel only allows images.
+    Users who send more than a few non-image messages will be banned from using the channel. 
+  args:
+    - ""

src/NadekoBot/data/strings/responses/responses.en-US.json

@@ -962,5 +962,7 @@
   "empty_page": "This page is empty.",
   "pages": "Pages",
   "favorites": "Favorites",
-  "tags": "Tags"
+  "tags": "Tags",
+  "imageonly_enable": "This channel is now image-only.",
+  "imageonly_disable": "This channel is no longer image-only."
 }