Add threading

[maybeanerd]

Based on elk-zone#2148

Here's my own ideas and notes to work with until I create the PR - after that i'll continue the discussion there

• next to send Button, have thread button
• when thread is active, show how many messages there are with a small number indicator
• show two button when threads are active: Back and Forward. These Switch between the actively edited message. When on latest, "forward" is a plus
• hijack the entire editor - whatever Message is active looks like a normal Message creation, has all tools
• allow stopping thread, warn about messages lost
• auto prefix sent Messages with 🧵 x/n
• publish button becomes "publish thread" or something

[maybeanerd]

For the start (or maybe forever), instead of allowing forward and backward inside a single Editor, just stacking editors might be the solution

Each previous Message doesn't have a publish or Threads Button anymore, but it can be "deleted" from the thread