Checking for "expired" sessions/logins on the client side

Comparing #12 with #13

Content

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In the proposed methods, the popups are modal to force the user to reload or close the page.

You should use your own CWebUser subclass as indicated below for full functionnality.

I haven't set up a test case to demonstrate the issue, but the following procedure should demonstrate the issue:

 
- Open a web page in your browser with a form relying on the YII_CSRF_TOKEN for submitting the data.
- Close the browser (with the reopen tabs functionnality active);
- Reopen the browser -> your form page should appear.
- Try to submit the form - submission should not work (if your browser did not reload the page).
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