Add a thread-mode

[Dehelssey]

We should provide a thread-mode so people can easily create a thread without having to answer to themselves at each post.

Either the GlitchSoc way : when thread-mode is activated, answer to self modal opens automatically). Either the Twitter way : you can write N messages and when thread is ready publish it all at once.

It's a request I see from a lot of newcomers. Even if we offer 5k characters, it appears it's easier to read (and share*) several shorter posts than one big fat novel 😄

• you can boost the part of the thread you agree with the most even if you don't agree with the whole thread for instance