Implement Ring username registration

AccountCreationWizard:
 - Users can now choose to register a username. They can also decide
   to do it later.

UsernameRegistrationBox:
 - New widget that allows to lookup usernames and see if they are
   available. It can also be used to register a username with the
   Register button.

AccountGeneralTab:
 - Display the Ring username with the UsernameRegistrationBox and allow
   registering one.

Change-Id: I0c01ee34dc3749aac8a000a0d574ea8561453cba
Tuleap: #997
[stepan.salenikovich@savoirfairelinux.com: improve lookup usernames
 code; prevent sending querries when not using blockchain]
Signed-off-by: Stepan Salenikovich <stepan.salenikovich@savoirfairelinux.com>