Make archived projects completely read-only

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/44788

EE port: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5227

To make respect read-only-ness:

• "New merge request" buttons/links
• "New file" button
• Blob "Edit"/"Replace"/"Delete"/"Web IDE" buttons
• Commit "Cherry-pick"/"Revert" buttons
• IDE button
• Emoji reactions

To do:

• Add specs
• Fix specs
• Add specs for emoji reactions
• Fix phrasing in UI
• Add documentation

/cc @jramsay