Consistency: make haml version of markdown field's control button row look and feel like Vue version

When you are commenting on a snippet or a commit, you see a slightly different markdown editor than when you are commenting on/editing the description of other things in GitLab:

Normal markdown editor	Commit/snippet markdown editor

The control buttons rail on the haml version (used for snippets and commits) should be adjusted to match the look and feel of the vue version.

• "Go full screen" button needs a tooltip
• "Go full screen" button should be pinned to the right edge, separated from the other controls
• There should be dividers present between some buttons

cf. app/views/shared/blob/_markdown_buttons.html.haml, app/views/shared/_md_preview.html.haml