Make it possible to disable update site regular fetches

In some situations like when the user is not admin, we do not want to launch the update check process. This should be manageable in the UI, in the general settings tab, with a new checbox whose label is "Check updates".

This checkbox should be checked by default. In case user doesn't have admin rights that are necessary, it should be unchecked and greyed/disabled, ideally with a message "You need to have administration right to activate updates".