Disable OTR chat logging by default

[dramb-lys]

OTR stands for "Off-The-Record", and by design supports perfect forward secrecy. Logging negates this.

Chelsea Manning was prosecuted using alleged OTR chat logs.

[TingPing]

Should be doable to spam /chanopt disabling text_logging and text_scrollback any time an OTR chat is started, ideally setting it back to previous state when finished (which is a bit tricky).