'Custom Server URL' Feature Parity

[wMarciano]

The ability to enter a custom server URL is a wonderful option, and one that makes the efforts of the stremio-service and the server-docker contributors worthwhile.

While this option is available on the Web, Desktop, and Android TV builds, it is notably missing from Samsung TV (and maybe other TV apps?), and even Android Mobile (as mentioned in #415). While Android Mobile clients can largely use the Web interface to get around this, many of the TVs cannot. So, if one's setup relies on a custom server for bandwidth/proxy/other reasons, the TV apps become unusable.

As such, it would be nice to make sure all platforms, especially those with no other options, have parity on this feature.