Validate username before submission

[AlbanBedel]

Currently the username is only validated after it has been submitted, this is not very user friendly. Some client side validation while the user is typing would feel a lot less frustrating. If possible it should not only check for the regex, but also if the username is still available. Similarly if the full name is mandatory that should be visually obvious.

[nickvergessen]

Mandatory full name has been added recently.
Currently the registration works without Javascript enabled, which until now i liked a lot as it makes sure any client can totally register with a simple webview.

We have to check at least the Desktop client and the mobile clients for files and talk to see that it doesn't break.