Add a Go back option to Web IDE

What does this MR do?

Adds a Go Back link to Web IDE.

The link points to the page where the WebIDE was triggered from. If no referrer is found, falling back to the Project URL.

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

There was some seemingly unnecessary code specifying min-height using calc that was causing issues and needed to have the calc() updated every time we added a new item.

I updated that and couldn't see any bugs or regressions in the layout. Please advise if I missed anything. (https://gitlab.com/gitlab-org/gitlab-ee/commit/75e0aaf6f4d063daf11a3da51433395ad8f6e261#c459364695ad302a6eea4dfa209a3ea071c73af5_635_635)

Why was this MR needed?

As part of improving the UX of jumping in and out of WebIDE, this provides an exit to the point they were before jumping into the Web IDE.

See https://gitlab.com/gitlab-org/gitlab-ee/issues/4592 for more details.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• 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 #4592 (closed)