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/device.rs

@@ -87,11 +87,11 @@ impl Device {
             nbf: time_now.timestamp(),
             exp: (time_now + *DEFAULT_VALIDITY).timestamp(),
             iss: JWT_LOGIN_ISSUER.to_string(),
-            sub: user.uuid.to_string(),
+            sub: user.uuid.clone(),
 
             premium: true,
-            name: user.name.to_string(),
-            email: user.email.to_string(),
+            name: user.name.clone(),
+            email: user.email.clone(),
             email_verified: !CONFIG.mail_enabled() || user.verified_at.is_some(),
 
             orgowner,
@@ -99,8 +99,8 @@ impl Device {
             orguser,
             orgmanager,
 
-            sstamp: user.security_stamp.to_string(),
-            device: self.uuid.to_string(),
+            sstamp: user.security_stamp.clone(),
+            device: self.uuid.clone(),
             scope,
             amr: vec!["Application".into()],
         };