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

migrations/20201001015631_not-null-github-avatar.sql

@@ -0,0 +1,35 @@
+
+-- Originally:
+-- ALTER TABLE users
+-- ADD COLUMN github_id bigint NOT NULL default 0,
+-- ADD COLUMN username varchar(255) NOT NULL default 'username',
+-- ADD COLUMN name varchar(255) NOT NULL default 'John Doe',
+-- ADD COLUMN email varchar(255) NULL default 'johndoe@modrinth.com',
+-- ADD COLUMN avatar_url varchar(500) NOT NULL default '...',
+-- ADD COLUMN bio varchar(160) NOT NULL default 'I make mods!',
+-- ADD COLUMN created timestamptz default CURRENT_TIMESTAMP NOT NULL
+
+-- We don't want garbage data when users are created incorrectly;
+-- we just want it to fail.
+
+ALTER TABLE users
+ALTER COLUMN github_id DROP NOT NULL;
+ALTER TABLE users
+ALTER COLUMN github_id DROP DEFAULT;
+
+ALTER TABLE users
+ALTER COLUMN avatar_url DROP NOT NULL;
+ALTER TABLE users
+ALTER COLUMN avatar_url DROP DEFAULT;
+
+ALTER TABLE users
+ALTER COLUMN username DROP DEFAULT;
+ALTER TABLE users
+ALTER COLUMN name DROP DEFAULT;
+ALTER TABLE users
+ALTER COLUMN email DROP DEFAULT;
+
+ALTER TABLE users
+ALTER COLUMN bio DROP DEFAULT;
+ALTER TABLE users
+ALTER COLUMN bio DROP NOT NULL;