Adds the settings pages to create OAuth2 apps also to the org settings
and allows to create apps for orgs.
Refactoring: the oauth2 related templates are shared for
instance-wide/org/user, and the backend code uses `OAuth2CommonHandlers`
to share code for instance-wide/org/user.
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
- Switch to use `CryptoRandomBytes` instead of `CryptoRandomString`, OAuth's secrets are copied pasted and don't need to avoid dubious characters etc.
- `CryptoRandomBytes` gives ![2^256 = 1.15 * 10^77](https://render.githubusercontent.com/render/math?math=2^256%20=%201.15%20\cdot%2010^77) `CryptoRandomString` gives ![62^44 = 7.33 * 10^78](https://render.githubusercontent.com/render/math?math=62^44%20=%207.33%20\cdot%2010^78) possible states.
- Add a prefix, such that code scanners can easily grep these in source code.
- 32 Bytes + prefix