Hide dropdown divider if user is issuable author

What does this MR do and why?

• Fix border showing in actions dropdown when viewing an issuable you authored

Screenshots or screen recordings

Various states:

	Before	After
User (not admin) viewing issuable they authored
User (not admin) viewing issue they did not author		No change
User (not admin) viewing issue they did not author, in a group/project they created		No change
Admin viewing issue		No change

How to set up and validate locally

Check out the issuable header dropdown in the different, semi-complicated states above 😅

UX Paper Cuts 16.4 → Pipelines and Jobs (#408231 - closed), UX Paper Cuts 16.5 → Cleanup (#424403 - closed)