Add isUsernameUsed to query methods

Fixes #13
Have fun @David!

src/graphql.rs

@@ -395,6 +395,17 @@ impl Query {
             .map_err(FieldError::from)
     }
 
+    async fn isUsernameUsed(context: &Context, username: String) -> FieldResult<bool> {
+        if !is_valid_user_name(&username) {
+            return Err(format!("{:?} is not a valid username", username).into());
+        }
+
+        Ok(sqlx::query(format!(
+            r#"SELECT user_name FROM users WHERE user_name="{}""#,
+            username,
+        ).as_str()).fetch_optional(&context.db).await?.is_some())
+    }
+
     async fn users(context: &Context) -> FieldResult<Vec<User>> {
         let rows = sqlx::query("SELECT * FROM users")
             .fetch_all(&context.db).await?;