Access Control: suppress login dialog (#471).

In a **default DM installation**: the browser's login dialog does not appear anymore when logging in via DM's login dialog failed. You keep staying in DM's login dialog. This applies in particular to Google Chrome.
Bottom line: in a default DM installation the browser's login dialog never appears. Regardless of browser (Safari, Firefox, Chrome).

However, in a **private DM installation** (dm4.security.read_requires_login=true) there is still an issue with logout: in Firefox and Chrome the browser login dialog still appears when clicking the Logout link. Remember: this dialog is a "dummy", you can press Cancel and you're logged out properly.

See ticket 471.