iframe: Allow Multiple iFrame Domains

Previously, we added a security header to prevent click-jacking called
"X-Frame-Options". This introduced an issue with people using osTicket in
iFrames on their websites. To mitigate the issue, this updates the security
header to allow the site to be framed from specified domains, if none
provided we default to 'self'. This adds a new field to General System
Settings called "Allow iFrames" where you may enter a comma separated list
of domains that the site can be framed on. This also adds a validator for
the field to validate the domains and ensure they fit the <host-source>
syntax from [Mozilla Developer
Docs](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/frame-ancestors#Sources).