Add more clippy checks for better code/readability

A bit inspired by @paolobarbolini from this commit at lettre https://github.com/lettre/lettre/pull/784 .
I added a few more clippy lints here, and fixed the resulted issues.

Overall i think this could help in preventing future issues, and maybe
even peformance problems. It also makes some code a bit more clear.

We could always add more if we want to, i left a few out which i think
arn't that huge of an issue. Some like the `unused_async` are nice,
which resulted in a few `async` removals.

Some others are maybe a bit more estatic, like `string_to_string`, but i
think it looks better to use `clone` in those cases instead of `to_string` while they already are a string.

src/db/models/user.rs

@@ -171,7 +171,7 @@ impl User {
     pub fn set_stamp_exception(&mut self, route_exception: Vec<String>) {
         let stamp_exception = UserStampException {
             routes: route_exception,
-            security_stamp: self.security_stamp.to_string(),
+            security_stamp: self.security_stamp.clone(),
             expire: (Utc::now().naive_utc() + Duration::minutes(2)).timestamp(),
         };
         self.stamp_exception = Some(serde_json::to_string(&stamp_exception).unwrap_or_default());