Add new function for custom config usage for Postfix and Dovecot (#5988)

Closes #5988

See merge request !1405

Resolve "system->cp users possible data loss warning unclear"

Closes #6056

See merge request !1413

Resolve "Enable http/2 for the panel (nginx)"

Closes #6065

See merge request !1421

Resolve "VApps VHost can not be disabled"

Closes #4992

See merge request ispconfig/ispconfig3!1420

Resolve "SSL for nginx apps vhost"

Closes #6017 and #6007

See merge request ispconfig/ispconfig3!1422

Resolve "Trigger junk move by Subject line impacts forwarded emails"

Closes #6051

See merge request ispconfig/ispconfig3!1409

Add missing http2 directive to HTTPS proxy protocol config in nginx vhost conf

Closes #6047

See merge request ispconfig/ispconfig3!1406

Resolve "postfix: references non-existent sasl_passwd file"

Closes #6048

See merge request ispconfig/ispconfig3!1407

jailkit bugfixes

Closes #6041, #6036, #6040, and #6042

See merge request ispconfig/ispconfig3!1401

'Enable receiving' should check disablesmtp

Closes #6044

See merge request ispconfig/ispconfig3!1402

Resolve "Jailkit init template have syntax errors"

Closes #6039

See merge request ispconfig/ispconfig3!1400

Resolve "BREXIT - Update country list"

Closes #6046

See merge request ispconfig/ispconfig3!1404

Resolve "Syntax Error in OpenVZ module"

Closes #6045

See merge request ispconfig/ispconfig3!1403

Use standard_index.html as filename for the template index file (#5897)

Closes #5897

See merge request ispconfig/ispconfig3!1398