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/routes/auth.rs

@@ -182,12 +182,12 @@ pub async fn auth_callback(
 
             User {
                 id: user_id,
-                github_id: user.id as i64,
+                github_id: Some(user.id as i64),
                 username: user.login,
                 name: user.name,
                 email: user.email,
-                avatar_url: user.avatar_url,
-                bio: user.bio,
+                avatar_url: Some(user.avatar_url),
+                bio: Some(user.bio),
                 created: Utc::now(),
                 role: Role::Developer.to_string(),
             }