[addons] switch general.addonupdates to an integer setting. Skins can use a button control for the setting instead of a
toggle.
[addons] adds 'never' as an option for the addon update setting
[confluence] use a button for the addon update setting to allow for 'never' as an option
[addons] adds RepoNeedsUpdate() to the add-on installer
[addons] change the 'Available updates' item to 'Check for updates' if the repositories need to be checked.