GUIDE: Grap the first 'x' seconds from a webm:

ffmpeg -i input.webm -ss 00:00:00 -t 00:00:0x -async 1 -c copy output.webm

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Guide: Transforming mp4 to webm on linux

ffmpeg -i <input.mp4> -c:v libvpx-vp9 -crf 30 -b:v 0 -b:a 128k -c:a libopus <output.webm>

Adjust -crf value to set quality. Lower value means higher quality at the cost of greater file size.

Guide on the visibility of topics (private vs public):

I define the content of a topic to be the following. Its posts, pinned posts, voice chat and files.

Public topic: 
Everything is public. Topic name, topic description and topic content.

Private topic: 
Topic name and topic description is still public. However, the topic content is private. Any content can only be viewed by users with access to that particular topic.  

Users with access are moderators and guest invited by a moderator. Moderators can grant guest access to whoever they choose, but also revoke this access again.