Auto merge of #2600 - JohnTitor:oauth2, r=jtgeibel

Update `oauth2` crate to v3

Release notes: https://github.com/ramosbugs/oauth2-rs/releases

This pulls two versions of `http` crate, it should be resolved once ramosbugs/oauth2-rs#103 is shipped. Otherwise, we can drop some (duplicate) dependencies. Also updates `url` crate to v2.

r? @jtgeibel

Author: bors