Allow additional login methods (SSO)

[mellowagain]

Probably allow login with the biggest VCS websites:

• [x] GitHub
• [x] GitLab
• [x] BitBucket

and maybe popular web services such as:

• [ ] Google
• [ ] Twitter

TODO on the implementation details:

• Save avatars
• Save SSH and GPG keys
• Verify CSRF tokens

[mellowagain]

Further investigation into this reveals that the protocol which should be used for this is called OpenID Connect. There are various crates which exist for this purpose, the most popular being openidconnect.

References for services:

• GitLab
• Google
• Twitter (OAuth 2.0)

[mellowagain]

GitHub has been implemented in 1f07ded. It is currently not visible on the login/register template but is already accessible via /sso/github.

[mellowagain]

GitLab has been implemented in 95febcb. It is currently also not visible but still accessible via /sso/gitlab.

[mellowagain]

BitBucket has been implemented in 47a0f23.