Add View Certificate menu item to account context menu
This adds a 'View Certificate' menu item to the account's context menu (part of #6096).
Viewing used ciphers and connection type (e.g. TLS version) would require changes to
python-nbxmpp I suppose. I tracked the code down to this line: https://dev.gajim.org/gajim/python-nbxmpp/blob/master/nbxmpp/tls.py#L389 . I think after that line one could invoke OpenSSL's
get_cipher_version() (see pyopenssl.org). Adding further connection info is not necessarily part of this MR.