Create new 'Hidden' featured mod capability

A discussion with Zock about how to run the various development branches and different levels of community testing resulted in a vision for the future that looks as follows:
- Forged Alliance Forever - The present master branch, played by the majority of people, and on the ladder and coop. We have this already.
- Next Version Beta - This will be kept up to date with develop day by day, and frozen at release branches for a week or two for fine tuning. This is open to anyone who wishes to play it.
- Open Balance Testing - This will be updated every time the inner core of the balance team settle on a change, every couple of weeks or so.
- Hidden Balance Testing - This will follow the balance branch on GitHub exactly, allowing the core balance team to instantly distribute changes by the minute. It will only be visible on the list to certain player IDs, chosen by Zock or whoever succeeds him as balance councillor.

That final one is where we need changes. We'll need support for a server check from the client when creating the buttons. I don't know how the featured mod list is set up on the server, but I imagine it's a table from which the buttons are populated.

We will need:
- Client support for checking if a featured mod is flagged as hidden
- Client support for checking if the UID is on the list of those allowed
- Server support for a flag and a list of UIDs
- Identity restricted API support for the balance councillor to be able to easily update the list
- Server support for detecting GitHub branch updates, automatically or through deploy: stable commands
- Identity restricted API support for updating which branch the server looks at for updates in the aftermath of a patch release

I am willing to help with this, but it is _way_ out of my skill-set. If someone wants to basically do it, but allow me to watch and help where I can, as a bit of a hand-held intro, that would be fantastic.