Change template variables to use theme variables.

Hey Dave, I'm putting this up early for feedback.

When using the theme locally, links for 'edit on github' and 'edit on bitbucket' are not show. There is also no way to add these links without copying the theme in locally and editing `breadcrumbs.html`.

This PR converts the `github_` and `bitbucket_` template variables to theme variables.

I know this is not the way RTD handles those variables though, and because they are pushed down into `theme.conf` it could be an issue.

*Note: Work In Progress*