Add option to periodically automatically upgrade selected extensions and keep them up to date

Similarly to how apps have the option to be auto-updated (enabled by default) on mobile phone operating systems (Android/iOS), so should Extension Manager periodically check if new versions of the installed extensions are available and upgrade them automatically.

The option to auto-upgrade could be global, per-extension, or a mix (the global option affecting all extensions and then being able to modify the setting for individual extensions as well; e.g. enable for all then go on some extensions and disable the option; We would need some "extension options" for that or, alternatively, if could be done with a blacklist that would negate the global option just for the listed extensions).

The current limit/issue this feature will encounter is "core" extensions that can not be upgraded and that block the upgrade of some extensions that depend on newer versions of the core extensions. This would simply mean that the extension will not be upgraded if it depends on the upgrade of a core extension. This would also effect the Standard Flavor, thus not allowing to keep XWiki Standard up to date until we get rid of the current model of installing extensions as core in the distribution, but this is OK for the moment. From a certain point of view, this could also be perceived as a good thing, since it will only mostly upgrade (keep up to date) the extensions you install and not the entire distribution.

Just FTR, how it works on Android: https://www.youtube.com/watch?v=pxvIrHLfKbg (also note that apps also get auto-upgraded in the background, which is not displayed in the video)