You can do this by going to settings > customize notifications > Android notifications and set Android notifications to silent. However, this provides no visual notification in Textra that it is silenced. It would be better to be able to do this in Textra and provide some visual indication in the app that the entire app is silenced so you don't leave it silenced for days at a time.

I would like to override all individual conversation settings to mute all conversations and then when I unmute all conversations, a customized notifications for each individual conversation goes back to what I have said it too. This is so I can globally mute Textra when I am sleeping but have to leave my phone on because I am on call for emergencies.