Per user E2BE enable/disable option

[olmari]

I'm one of the admins at Hacklab Finlands matrix-server, and we have variety of users some wanting all the encryption enabled and some couldn't care less for that, so:

Introduce user configurable E2(B)E flag that user can enable/disable with talking to bot. Should go both ways, if global default is set disabled user can enable it for oneself and vice-versa if global default is set enabled uset can disable it oneself.