Element config.json

Initially copied from https://develop.element.io/config.json, but modified.

Placement:

Flatpak: `~/.var/app/im.riot.Riot/config/Riot/config.json`
  * or `~/.var/app/im.riot.Riot/config/Element-PROFILENAMEHERE/config.json`
* Windows: `%APPDATA%\Element\config.json`

Labs features: https://github.com/vector-im/element-web/blob/develop/docs/labs.md