Update "Admin", "feed's related item" & "Goto Dashboard" button icons

[theskumar]

• 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.

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Test Steps

• [ ] View dashboards of different users, staff, contracting etc.
• [ ] check profile page with access to admin
• [ ] Check related item link in the activity feed.

[wes-otf]

Tested, all looked good just a weird thing when I was either Contracting or Finance is that the Admin option would show up on my dashboard, but not on my profile settings. Also couldn't access it and would return a ton of errors when selecting it (Finance admin button also didn't have the icon, but prob doesn't matter):

[frjo]

@wes-otf @theskumar That was an old bug where we missed to check perms.wagtailadmin.access_admin on these dashboards. I have fixed that in the PR now.

[wes-otf]

Looks good on test!