Note worthy changes

• allauth.headless now supports the is_open_for_signup() adapter method. In case signup is closed, a 403 is returned during signup.
• Connecting a third-party account in HEADLESS_ONLY mode failed if the connections view could not be reversed, fixed.
• In case a headless attempt was made to connect a third-party account that was already connected to a different account, no error was communicated to the frontend. Fixed.
• When the headless provider signup endpoint was called while that flow was not pending, a crash would occur. This has been fixed to return a 409 (conflict).
• Microsoft provider: the URLs pointing to the login and graph API are now configurable via the app settings.