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/api/admin.rs

@@ -427,7 +427,7 @@ async fn update_user_org_type(data: Json<UserOrgTypeData>, _token: AdminToken, c
         }
     }
 
-    user_to_edit.atype = new_type as i32;
+    user_to_edit.atype = new_type;
     user_to_edit.save(&conn).await
 }