Create a 'minimal' variant of GithubPermalink.

[dwjohnston]

The current Github Permalink block is very much based on what we see in Github.

This contains too much superflous content - usually what we want is just a codeblock. It should still be able to link back to Github though.

I think the best thing is going to be subtle Github Octocat icon that serves as the link, and a chevron to expand the full details.

The existing Github Permalnk can be extended - and should have a 'isInitiallyExpanded' property.

The global configuration should have 'initiallyExpandGithubPermalinks' to determine the default behaviour of isInitiallyExpanded. This default should be true.