Update "Admin", "feed's related item" & "Goto Dashboard" button icons (#3922)

- Rename "Apply Admin" to "Admin" 
- Adds "Admin" icon to the user's profile page along with the
"Dashboard" link, makes it easier to goto admin without going to
dashboard everytime first.
- Update icons for "dashboard" and "Admin" to use heroicons.
- Update activity feed related link icon + it's visuals
- Remove `arrow-head-pixels--solid` icon from the sprite as it's no
longer in use anywhere.


Co-authored-by: Fredrik Jonsson <frjo@xdeb.org>