1.2.0-pre Not Very Simple Anymore

Pre-release v.1.2.0

• @kylesands correctly pointed out that the code introduced in 1.1.0 doesn't work properly: when clicking Submit without having a valid session, all local storage will be deleted as well (because Submit has that side-effect). So we need to address this case before clicking submit, and this can only be accomplished by hooking up to the correct event and properly exporting the session expiry date, because phpBB forcefully hides the session cookie from the JavaScript sandbox.
• Therefore, this is not a "very simple" extension any more. Just "simple".
• Other minor bug fixes (kudos again to @gvp9000).
• Note for the pre-release version: it still includes far too many debug messages.