Misc fixes + improvements

[tadhgboyle]

• Ability to change user language + timezone from staffcp
• Display active user template in staffcp
• Add "last seen" to user popup, and add tooltips for exact date of last seen + registered
• Limit online staff and users widgets to only showing 10 users
• Add ability to create header in dropdown menus (will be utilized by members module when merged)
• Use .gitkeep instead of placeholder.txt files
• Add og:image uploader (closes Add OG Image upload section #3267)
  • Allow setting specific og:images for each page via SEO config
• Remove old fomantic toast assets
• Remove old datatables plugin files
• Improve email validation error message
• Add avatar_url to UserInfoEndpoint
• Add fallback message to LatestPostsWidget
• Cleanup root directory (move files into .github or dev)
• Allow disabling auto language feature & improve detection (closes Switch off auto language selection #2861)
• Fix latest posts profile tab no posts message not displaying

outdated