Web IDE markdown preview

What does this MR do?

• Adds Editor Tabs to switch between Edit and Preview
• In Review Mode it will switch to Review as the title (Main reason is the Icon Button Bar on the right)
• Moved the bottom buttons to the top right and made them icon buttons
• Introduces a generic Content Viewer in Vue which is then used in the Markdown Preview.

Are there points in the code the reviewer needs to double check?

• Basic Setup of the generic content viewer (Next version will do dynamic returns)
• Setup for determining which previewer should be used

Why was this MR needed?

To enable Markdown Preview

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?

Closes gitlab-org/gitlab-ce#44843