Allow uploading of custom scene covers (#262)

* Refactor common code

* Further refactoring

* Add UI support for changing scene cover image

* Add backend support for changing scene screenshot

graphql/schema/types/scene.graphql

@@ -51,6 +51,8 @@ input SceneUpdateInput {
   gallery_id: ID
   performer_ids: [ID!]
   tag_ids: [ID!]
+  """This should be base64 encoded"""
+  cover_image: String
 }
 
 input BulkSceneUpdateInput {