Refactor and const IdentityManager.

See https://invent.kde.org/pim/kleopatra/-/merge_requests/16#note_427791

As per the comment, mailIdManager has been moved from a class static
variable into a file static function.
This revealed that it is only required to get hold of the identity
associated with a certain mail sender address. Thus the class code using
the IdentityManager has been refactored into the same function and it no
longer returns an IdentityManager, but instead directly returns the
identity requested, or the default identity if a mail address has no
associated identity.