Application.CertificateManager: Check locally pinned certs for equality

When checking if a certificate is pinned locally (i.e. when GCR support
is unavailable), ensure the presented cert is identical to the stored
cert.

Fixes #866
3 jobs for mjog/866-self-signed-certificates in 22 minutes and 7 seconds (queued for 1 second)
Status Name Job ID Coverage
  Build
passed fedora #795815

00:04:07

passed ubuntu #795816

00:07:33

 
  Test
passed flatpak #795817
flatpak

00:14:33