Improve various commands by making use of threads

[PKlempe]

Now that Discord supports Threads, they can be used to make some commands more useful.

Proposed Improvements:

• !suggest: For every suggestion received, create a corresponding thread in which users can discuss it. This removes the need for a dedicated suggestion discussion channel and keeps everything in one place.
• !modmail: The same goes for Modmails but with the addition, that the user sending the modmail, should automatically be added to the private thread. This way, moderators have a dedicated place where they can talk with the user that no one else can access. In the end, the thread can simply be archived by the mods so that it doesn't take up space in the channel list.
• !report: A thread for every report received so that the moderators can talk about how they should handle a specific situation.