Open photos embedded in preview cards in the image viewer (#1966)

* Open photos embedded in preview cards in the internal image viewer instead of opening the browser

* Enable toolbar for single image viewer

* Apply review feedback

app/src/main/java/com/keylesspalace/tusky/fragment/ViewImageFragment.kt

@@ -99,9 +99,9 @@ class ViewImageFragment : ViewMediaFragment() {
             url = attachment.url
             description = attachment.description
         } else {
-            url = arguments.getString(ARG_AVATAR_URL)
+            url = arguments.getString(ARG_SINGLE_IMAGE_URL)
             if (url == null) {
-                throw IllegalArgumentException("attachment or avatar url has to be set")
+                throw IllegalArgumentException("attachment or image url has to be set")
             }
         }