Support GitLab and Bitbucket repositories

Support GitLab and Bitbucket repositories

site.git accepts a provider option (github by default, plus gitlab and bitbucket). The navbar icon link and page source links now point at the configured provider with its own brand icon. Self-hosted instances can set git.url to their instance URL.

ctx.getFileGitHubUrl() is renamed to ctx.getFileUrl(), the old name still works but is deprecated.