Login form is submitted twice if the login button is double clicked - and may create user profiles twice depending on the authenticator

How to reproduce:

• go to the XWiki login form
• fill in credentials (wrong ones for highlighting this problem, but it does just the same for correct credentials)
• double click login button
  • or click as many times as you want

Expected result:

• the authentication process is launched a single time, and the login button is disabled when the form has been sent and until an answer is received from the server
• since it's a post request, this is aligned with the principle of a post form - unless it's OK to execute the same action multiple times the button should be disabled by the application instead of expecting the user to stop clicking.

Actual result:

• on the server, the log "Authentication failure with login [...]" is displayed in logs, showing that multiple login requests are processed on the server, one for each click.