WIP: Frontend for "Open Web Terminal from attached CI runner in Web IDE"

What does this MR do?

This is the Frontend for #5426 (closed).

Notes on commits:

• This branch contains some CE cherry-picks since they were not merged in at the time. Now we're waiting for the ce-to-ee merge to clean up this branch. These commits are labeled "(cherry)".
• This branch also contains some commits which need to be ported to CE. These commits are labeled "(ce)".

Screenshots

Demo:

Invalid .gitlab-ci.yml:

Invalid runners:

What are the relevant issue numbers?

• #5426 (closed)

TODOs

UX:

• "Scroll up" and "Scroll down" buttons on terminal
• Add URLs for help pages
• Adjust vertical height of terminal
• [ ] Fix line wrapping of terminal (when resizing pane lines do not correct their wrapping) Out of scope for this iteration.
• Fix white scrollbar

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?