Draft: Configurable inactivity timeout for OMEMO
The idea of this MR is to let the user set after how many days without an OMEMO message another device will be considered inactive, instead of a fixed number of messages. This should make the mechanism easier to understand, and decide one's forward secrecy balance.
This draft currently adds only the configuration parameter for the plugin and UI to set it, it does not change the actual inactivity handling yet.