Message bar

[binwiederhier]

This implements a WhatsApp style message bar and a publish dialog just like in the web app. See binwiederhier/ntfy#98

Remaining TODOs:

• Weird spinner location in publish dialog after send is pressed
• Make large uploads cancelable
• Text color in message bar needs to be slightly darker
• Make sure dark mode and dynamic colors look alright#
• Self-review
• Test, test, test

[binwiederhier]

Supersedes #45