Fix Issuable container element selector for shortcuts handling

What does this MR do?

This MR fixes ~"broken master".

It fixes the selector used to get Issuable body container from the DOM so that it can work in both Issues/MRs and Epic (with both having slightly different DOM structure with Epics being a Vue app).

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?

Closes #8511 (closed)