Draft IG publication shows list of branches

[litlfred]

When in draft publication mode for the main branch of REPONAME (which is accessible at ORGNAME.github.io/REPONAME) it would be good to have in the yellow status box on the home page of the IG, where it links to the list of published version, a list of the feature branches (non-main, non-gh-pages) branches.

When in the draft publication mode for a feature branch, FEATURE, of a repo REPONAME (which is accessible at ORGNAME.github.io/REPONAME/branches/FEATURE), it would be good to have a link back to the draft publication of the main branch.

@costateixeira @ritikarawlani