Feature: Reopen resolved threads

[janreppien]

Sometimes we close a thread and realize later that the issue discussed in it is not resolved. It would be handy to be able to reopen the threads so that the discussion can be complete traced by new people joining. Currently, we open a new thread and if necessary copy old messages by hand.
Overall, the ability to add comments to a section in the editor is really awesome, and speeds up communication immensely.

[areknawo]

Currently, thread's relation to the commented section is removed when it's resolved.
So a thread could be reopened but it won't be associated with the selected content anymore (and thus, available only from the side panel). That's why it's currently disabled and why it'd be a bigger change to fix that. Will consider it though.