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/auth/mod.rs

@@ -58,7 +58,7 @@ where
     match res {
         Some(result) => Ok(User {
             id: UserId::from(result.id),
-            github_id: result.github_id as u64,
+            github_id: result.github_id.map(|i| i as u64),
             username: result.username,
             name: result.name,
             email: result.email,