enable BccSelf by default (#3612)

* enable `BccSelf` by default

enabling `BccSelf` improves user experience as
it is easier to set up another device
and ppl will also see "all" messages in other user agents directly.

for uncounted user problems, after diving into the issue,
the resulting device was "turn on BccSelf".
disabled `BccSelf` was probably the the number one single reason
of user problems.

main drawback of the change are potentially double notifications
when using a shared account and having another mail app on the same device.
however, we meanwhile do not recommend shared accounts at all,
the issue is also fixable by the other mail apps (as done by K-9)
and could be even regarded as a feature (you can decide which app to use for ansering).
but at the end the drawback is probably much smaller than the issues reported above.

* adapt tests to `BccSelf` enabled

* update CHANGELOG

src/config.rs

@@ -55,7 +55,7 @@ pub enum Config {
     Selfstatus,
     Selfavatar,
 
-    #[strum(props(default = "0"))]
+    #[strum(props(default = "1"))]
     BccSelf,
 
     #[strum(props(default = "1"))]