OAuth integration and Workspace starts with Bitbucket Server v7.20

[jldec]

This is the first step toward enabling integration of Gitpod with self-hosted BItbucket Server and Bitbucket Datacenter installations.

With their v7.20 release, Bitbucket Server and Datacenter now support OAuth2.

• Enable OAuth logins between Gitpod and a Bitbucket Server v7.20 Server instance
• Extend the context parser and enable workspace starts with prefixed URLs

[gtsiolis]

@jldec see also #1965.

[jankeromnes]

(FYI, undoing the latest title change to "pre" because it doesn't look intentional 👀)