Migrate the sender identity settings via the central place

v2: Fix missing call to QSettings::endArray(). Thanks to Caspar for noticing a
regression, and blame me for not testing this. Sorry.

v3: Do it reasonably, not via duplicated calls to endArray. Thanks to Caspar for
calling me out on this crappy code.
Remove correct settings item when changing hosts

This code was never updated when we switched from tracking certificate chains to
just storing the key of the leaf SSL certificate. The old key should have been
renamed to include the "obs" prefix.
XtConnect: store a correct key
Make it clear that this settings key is obsolete now