Updated to vue(tify) 3

Largest changes:

• MDI icons now need to be imported
• text attribute of <v-btn /> removed
• dark attribute changed to theme="dark"
• Rewrote admin-app-bar. The old style had problems with the icons.
• Replaced video-bg with VideoBackground. Old one isn't compatible with vue3.
• Layout selector is now in router, not in the App.vue.
• Removed linter. New defaults change every line, which isn't required now.