Implement OAuth support for Github login & package management

[Geod24]

Package management is currently complicated: there's a lot of duplication involved between Github and the registry, and e.g. while transferring rights for a repo is easy, doing it for a package requires work from Sonke.
Additionally there is no way to collaboratively manage a package. The only way would be to share a user account, which includes sharing the password. While possible, we all know the shortcomings of this.

Given a large majority of packages are currently hosted on Github, it would make sense to allow users to link their Github account with their registry account. Adding support for organization would also be needed, but would then allow contributors to orgs, such as dlang or dlang-community to collectively manage the packages they have write access to.

[schveiguy]

In absence of a link to github, just being able to have multiple owners of a project would be useful. Not really as good, but maybe simpler to implement in the short term.

[SingingBush]

this sounds good. I work on both ddbc and hibernated but did not create the repo and cannot do any of the maintenance from code.dlang.org side of things. If we can maintain it as an organisation on github that would be great.