feat(avatars): profile images

Getting there! Feel free to start looking @shane-tomlinson @pdehaan.

Still needs:
- [ ] profile-server client
- [ ] progress indicator screen
- [ ] unit tests for camera view
- [ ] unit tests for cropper widget
- [x] circular overlay on cropper
- [ ] functional tests
We should probably save the `BIG_STUFF` style for constants. I would rename `DISPLAY_LENGTH` to `DEFAULT_DISPLAY_LENGTH` and then say `this.displayLength = options.displayLength || DEFAULT_DISPLAY_LENGTH`. Ditto for `EXPORT_LENGTH`.