[omemo] Add optional setting to enable encryption by default

Hi and thanks for a great plugin!

I couldn't find any existing issues on this.

What I am missing is a setting in the plugin's options to require encryption by default, so I don't send unencrypted messages unintentionally. In an older version of gajim/the plugin, sending unencrypted messages was very noticable due to the feedback from the UI, but now I can go a long time before noticing the unlocked lock icon.

It would be great if I could make encrypted messaging the default, requiring my disabling it manually for each chat every time if I want to chat unencrypted.

Thanks for reading!