disable bcc_self on new installations

[r10s]

• bcc_self should be disabled on new installations to mitigate annoyances on first time installation, eg. this would avoid double-notfications.

• when an autocrypt-setup-message is really sent or received successfully, it is enabled implicit.
  EDIT: no, see comment below

• for existing installations, bcc_self should not be changed

[r10s]

when an autocrypt-setup-message is really sent or received successfully, it is enabled implicit.

no. we should not try to be too smart here. enabling/disabling settings after first-time-installation might be even more annoying to the user.

instead of implicitly changing things, a message should be added to the new device-chat when multi-device is detected in some way. to keep things simple for now, the message should just explain what to do, having some click-here-to-edit-x seems to be a bit out of scope - and maybe not needed at all