Preview markdown in multi-file editor

The standard blob viewer will be used for non-editable files like images following https://gitlab.com/gitlab-org/gitlab-ce/issues/38355 but we should allow users to preview markdown like they currently can.

Proposal

Add an option to the interface to allow a markdown buffer to be rendered and previewed

Design

• tabs + "Action Icons" bar
• each action item has a tooltip describing the funcion (for example Delete file)
• "Edit" tab is always visible, even if "preview markdown is not available
• lock icon uses unlock and lock states

bottom now looks like:

Icons:

• Download file --> download
• Check out file locally --> monitor-lines
• Blame --> blame
• History --> history
• Permalink --> link
• Lock --> lock