Add/use option: SCFG->System->Security->Create Self-signed Certificate

Default to false / No. Maybe this should default to true / Yes until a
certificate (e.g. from Let's Encrypt) is successfully provisioned?

Anyway, this is just a quick-hack to address the issue where a sysop's
CA-sigend certificate may be overwritten with a self-signed certificate if
for some reason Synchronet can't find/read the ssl.cert file.

See issue #881.