Right now, when the user clicks to synchronize a site we only invalidate the downloaded data so it's redownloaded the next time he accesses the module.

As the name "synchonize" implies, we should also execute the synchronization processes only for that site. Since we are executing a partial synchronization (only one site), the last execution time shouldn't be affected so it's possible that the app executes the site sync and the "all sites" sync at the same time.

Some things to take into account:

• The documentation of the cron delegate already specifies that it can pass a siteId to the execute function. SCORM and quiz sync functions already support both options, all sites or single site, so they can be used as an example.
• We should show a spinner or something similar until all the processes are done. Right now we block the user using mmUtil#showLoading, we shouldn't do that.
• Should this skip the queue of cron processes?