Configure auth footer links through Riot config

[TheJJ]

Our organization needs a imprint and privacy statement right on the login page, and some links to organization pages as well.

I've added a possibility to customize those links through the config.json:

    "branding": {
        # this is already upstream:
        "welcomeBackgroundUrl": "themes/org/bg.jpg",
        "authHeaderLogoUrl": "themes/org/orglogo.svg",

        # these are now added by this pull request:
        "authFooterLinks": [
            {"text": "Our Organization", "url": "https://best-org-ever.edu"},
            {"text": "Some subdepartment", "url": "https://subdepartment.best-org-ever.edu"},
            {"text": "Impressum", "url": "/legal/impressum.html"},
            {"text": "Datenschutz", "url": "/legal/datenschutz.html"},
            {"text": "github", "url": "https://github.com/vector-im/riot-web"}
        ]
    },

Fixes #9127.

[jryans]

Thanks for the contribution! It looks reasonable overall, and it addresses something already requested by others. 😁

Please add a line to the config.json section of the README about this new option.

[jryans]

Great, this looks good to me. 😁

[FilexCZE]

Hey, still this working?

[t3chguy]

@matyasvane yup.

https://chat.mozilla.org/#/welcome