[addons] add support for 'never' updating or checking for updates

3 commits here.  The first switches the existing boolean setting 'general.addonautoupdates' to an integer setting 'general.addonupdates'.  There's no upgrade path here, so it will default to "auto update".  @Montellese I don't think there's an easy upgrade path as the previous setting was boolean and is now gone?  I don't think we can update boolean -> int in this way, right?

The second adds 'never' as an option.  This essentially eliminates the repository update jobs.  The repositories can then ONLY be updated via the context menu.  I could potentially change this to force an update when the user clicks on "Get Add-ons".

The last adds the change to Confluence.  Everything still works with a toggle button (i.e. other skins) but the never option is unavailable here.

One question is whether to split the strings up to use 'Auto updates: %s' instead of the current setup.  I don't really see a major advantage in this though.  Alternatively I could split the button into label1+label2 I guess.  One concern I have is space on non-English languages.