Visual Revisions: remove show template option and pathway to opening Styles

[annezazu]

When looking through revisions for a post, I noticed I can turn on "Show Template" and, from there, the Styles icon is visible and I can change the styles of the entire site. Styles should not be available at all from Posts.

Replication steps:

1. Open a post with revisions and the latest of GB installed (trunk).
2. Select the option to show template.
3. Click "Styles" icon and notice you can change things.

Video:

revisions.with.styles.access.mov

cc @ellatrix