Fix inadequate certificate validation

This is a part of some changes that I had been working on.  In light of the recently created issue 6072, I'm going ahead and submitting this part now, since it is likely related to that issue.  (Note, however, that I haven't contacted the source referenced in the issue, so I can't say with certainty that this commit addresses the specific problems encountered there.)

Proper host name validation was not being performed for certificates
kept in the local keystore.  If an attacker could convince a user to
accept and store an attacker's certificate, then that certificate
could be used for MITM attacks, giving the attacker access to all
connections to all servers in all accounts in K-9.

This commit changes how the certificates are stored.  Previously, an
entire certificate chain was stored for a server (and any of those
certificates in the chain were available for validating signatures on
certificates received when connecting).  Now just the single
certificate for the server is stored.

This commit changes how locally stored certificates are retrieved.
They can only be retrieved using the host:port that the user
configured for the server.

This also fixes issue 1326.  Users can now use different certificates
for different servers on the same host (listening to different ports).

The above changes mean that users might have to re-accept certificates
that they had previously accepted and are still using (but only if the
certificate's Subject doesn't match the host that they are connecting
to).

This commit modifies AccountSetupBasics so that it now calls
AccountSetupCheckSettings twice -- once for checking the incoming
settings and once for the outgoing settings.  Otherwise, an exception
could occur while checking incoming settings, the user could say
continue (or the user could accept a certificate key), and the
outgoing settings would not be checked.  This also helps with
determining if a certificate exception was for the incoming or
outgoing server, which is needed if the user decides to add the
certificate to the keystore.