Web Terminal FE

What does this MR do?

This branch finishes up the FE for #5426 (closed).

Please note:

• CE Port https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23626
• Changelog is not needed and has already been added in backend MR.

TODO:

• Fresh screenshots
• Finish up unit tests
• Create CE port

Screenshots

Demo	Invalid config	Invalid runners

What are the relevant issue numbers?

#5426 (closed)

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
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• 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?
• Security reports checked/validated by reviewer