Refactor tags services and added tests

Implemented some functionality which will allow the service
configuration of the app to be split among the features for readability
and testability.

src/database/schema.rs

@@ -33,9 +33,4 @@ table! {
 
 joinable!(tagged_photo -> tags (tag_id));
 
-allow_tables_to_appear_in_same_query!(
-    favorites,
-    tagged_photo,
-    tags,
-    users,
-);
+allow_tables_to_appear_in_same_query!(favorites, tagged_photo, tags, users,);