Add Xbox as Predefined Site

[craeckor]

You can actually still view Xbox-profiles with https://xbox.com/play/user/{gamertag}.
Would be nice, if this gets implemented.

[tvx-matt]

I am personally going to attempt to implement this in my own instance, if it goes well, I will suggest it to the team.

[tvx-matt]

@craeckor I have made a pull request #855 to implement this. I would also personally benefit from this.

[tvx-matt]

I also think that Threads should be added, as a predefined site and a FontAwesome Icon