Adding Config Settings for All Configurable Riot Settings

[gary-kim]

List of configuration in Riot: https://github.com/vector-im/riot-web/blob/develop/docs/config.md.

There are separate issues for some of these settings:

• integrations_ui_url, integrations_rest_url, integrations_widgets_url: #18
• showLabsSettings, features: #2

[Thatoo]

It would be great to add https://github.com/vector-im/element-web/blob/develop/docs/jitsi.md too

{
  "jitsi": {
    "preferredDomain": "your.jitsi.example.org"
  }
}

[gary-kim]

@Thatoo We already have Jitsi configurable: https://github.com/gary-kim/riotchat/blob/78eca86476c8b6bdd5bd572beba679a27a3a970a/src/components/AdminSettings.vue#L121-L134