Allow Setting Private Key / Certificate for Enabling SSL

[jaruba]

Currently Powder Web only supports self-signed certificates, so these settings are a must in order to use a custom SSL certificate.

[Tekka90]

Please note that this can be workaround using Containerisation (With letsencrypt for example)

[jaruba]

Any http proxy server can work around this really, but as Powder Web is a web server, and as it already supports creating a self signed certificate, it should support legitimate certificates also.