Add "Check for Update" button to error alert when DB version is newer

This adds a "Check for Update" button to the "Your database is newer...." error alert that will either
  * launch the ZSA updater (haven't tested this actually)
  * launch an update check for Zotero add-on and open up the Add-ons -> Available Updates tab in Firefox .

It works quite nicely IMO, but for some reason update checks this morning have been quite slow (~15 seconds). It's not just via this method - through the add-on manager as well. If this is temporary then I guess it's not a big deal, but otherwise we may want to add a progress indicator.