Make Webport configurable

[linuxhelf]

This change allows to change the port under which greenmail-http can be reached by setting the property uk.co.bigsoft.greenmail.web_port

For this to work, the frontend now uses the current browser url.
This means that if the firewall permits, the frontend can now also be used from a different computer, since the backend url is no longer hardcoded to localhost:7000.

This would break frontend development with yarn start, however setting "proxy" in package.json configures yarn to redirect all unknown request to this url.
Therefore yarn start still works and there should also no longer be any CORS problems.

[davidnewcomb]

Sounds good to me.

Please can you update the README and add a section under "Run with Access-Control-Allow-Origin" to describe this new option.

[linuxhelf]

I've updated the readme, please review