Allow to skip sending thread replies to the main channel

[zdumitru]

Is your feature request related to a problem? Please describe.
New threads functionality is great, but when multiple threaded discussions are ongoing, the main channel becomes a disaster where it is not possible to understand what comes from where.

Based on the experiese, authors want only 10% of the threaded replies to end up in the main channel so that all main channel members are explicitly notified about it.

Describe the solution you'd like
A checkbox close to the threaded message reply field saying "Send the reply to the main channel" or something similar.

Describe alternatives you've considered
A setting allowing user to decide whether it is possible to not send threaded replies to the main channel.

Additional context

[engelgabriel]

Related #129

[engelgabriel]

@zdumitru thanks for the feedback. We are planning to do something like this on the next version of threads. Please, let's concentrate all the feedback and ideas on a single issue: #648

Thanks!