Notify everyone up on comment thread

[Betree]

Today, when replying to the reply of a comment, the comment's author doesn't get notified (only the direct replies do). The notification should go up the tree and notify everyone above.

[Betree]

Specs proposal

• [ ] Add a thread column on Comments (string)
• [ ] When creating a comment, the value for thread should be:
  • nil if it's a root comment
  • #{reply_to.id} when it's a reply to a root comment
  • #{reply_to.thread}-#{reply_to.id} for other replies Example: 42-67-88
• [ ] In the migration, generate thread for all existing comments
• [ ] When querying for the accounts to notify, look at the first id from thread (the root comment) and notify everyone

Storing the thread like that has multiple benefits:

• It allows us to review who should be notified in the future
• It gives an indication of the current depth of the comment (useful for https://github.com/CaptainFact/captain-fact/issues/38)

It has one major downside: it's a "soft" relation, meaning that if we delete/edit one of the root comments children won't be deleted.