UserSettings need to be rationalised

[jackjansen]

Choices that are purely local, such as which type of point cloud representation to use, should be stored in the per-user configuration. The same for the username (which is now really a nickname, not a username), which microphone to use, which camera to use.

And I'm sort-of wondering whether this also holds for settings like tiling and quality levels of point clouds, but maybe leave that for later.

Note that there is a potential problem with built players: the config-user.json file is stored inside the player folder (or inside the .app bundle on Mac), and this may not be writeable.

Relevant to #123 (actually sort-of implied there as well), but also to #122 and #181.

For #181 because it will allow us to keep all settings for all participants in config.json and config-user.json, which makes it much easier to run repeatable experiments.