Avatar edition modal does not have the proper modal markup

When changing the avatar of the user with a keyboard, we can open the modal but then two problems happen:
1. The focus is outside the modal, in the background content.
2. The user is not notified that the modal is shown.

We need to make sure the focus is brought into the modal and locked in here while it's open.

In order to do that, a possible solution is to use the ARIA markup: `role=modal`.