Update profile screen for tech preview

[robertlong]

You should have a few avatars to pick from in the profile screen. These should be populated from a list stored in a JSON file. The JSON file should also store urls to thumbnails for each avatar. Each url should be an mxc url to an asset stored on the matrix homeserver to avoid uploading/downloading duplicate assets.

[robertlong]

On second thought we're probably going to launch with just the X and Y bot avatars being randomly assigned to you when you join the room. We want to take our time to introduce any avatars and work with creators to make sure they meet our quality standards before we go ahead with custom avatars.

So, with this in place and no real way to create custom avatars at this time, I think we should hide the avatar upload in the profile for now.