Disable Encrypt button of Notepad if invalid recipients are selected

This prevents "Unusable public key" errors if a certificate that cannot
be used for encryption (e.g. expired, revoked, or otherwise invalid
encryption subkey) or a group with such a certificate is selected as
recipient.

(cherry picked from commit 20fbbb305345cf48ba2f15024349726e89c9ed37)

• GnuPG-bug-id: T6742