Thread creation

[titouanfreville]

Discord does not manage thread but we would often wish to have this feature in devcherchecolegue. So we should make the bot do it :)

As a user:

• I should be able to start a thread response to a message.
• I should be able to answer the thread
• I should be notified when a new message is posted in a watched thread

A thread is watched if I explicitly watch it through a bot command OR when I participate in it.