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/mod.rs

@@ -1,10 +1,8 @@
 use bcrypt::{hash, verify, DEFAULT_COST};
 use diesel::prelude::*;
 use diesel::sqlite::SqliteConnection;
-use std::{
-    sync::{Arc, Mutex},
-};
 use std::ops::DerefMut;
+use std::sync::{Arc, Mutex};
 
 use crate::database::models::{Favorite, InsertFavorite, InsertUser, User};