General cleanup: fix some bugs, some refactoring (#65)

* Merged mod file upload in version creation, mod creation and
  version file add to one function;  This makes sure that they are
  consistent
* Made some fields on `User` optional: `github_id`, `avatar_url`, `bio`.
    * We may not want to publicly show the `github_id` to everyone
      with access to the API
    * If we allow non-github users, some of those fields would be
      invalid; some oauth providers may not have avatars or bios
* Made CORS origins should configurable
* Made `--reconfigure-indices` and `--reset-indices` exit after
  completion instead of starting the server

src/database/postgres_database.rs

@@ -43,7 +43,7 @@ pub async fn run_migrations(uri: &str) -> Result<(), sqlx::Error> {
 
     for migration in migrator.iter() {
         if migration.version > version {
-            let elapsed = conn.apply(migration).await?;
+            let _elapsed = conn.apply(migration).await?;
         } else {
             conn.validate(migration).await?;
         }