Add support for environments built in AFrame

[rvdleun]

Goal

Let the user pick custom AFrame environments

Description

AFrame is incredibly easy to pick up. People can put something together with just HTML and a bit of Javascript if they want. We should make use of this.

At the moment, On/Office is serving a static HTML file with the splashscreen and the AFrame <a-scene>. This item is about letting the user pick a custom environment which will replace the default <a-scene>.

This would replace the 'Change 360 image' button on the desktop application. Rather, the user should be able to point to an archive file that will have the AFrame scene, plus eventual assets.

Next to this, each AFrame scene could also have a number of variables that the user can set. For example, the current scene should be available in the same format, and the user should still be able to change the background image.

For builders, we'll likely also want to have a template / repository for them to be able to start building a scene.

Later on, it would be neat to have a 'repository' where builders could add their own environments for others to share.

Deliverables

• Make the a-scene that is served dynamic, instead of hard-coded.
• Convert the current scene into a dynamic one and have it set as the default environment
• Let the user pick an environment
• Have a starter's template for builders