perform update via AJAX and report progress of update as it continues

[diosmosis]

Currently, when Matomo is updated and needs to run DB migrations, it will do so upon loading a Matomo page. This means the page won't finish loading until the update completes, and the user will have no idea what the progress is.

This UX is not ideal, it would be much better to load the page, keep it functional, and report on the progress of the update to the user so they know when it occurs.

Notes:

• if a user decides to navigate to another page while the update is in progress, the update should still continue in the background.
• if an update is non-trivial (eg, for a major Matomo version), the update should warn the user it may take a while to complete, and explicitly ask the user to start the update. We don't want the user to accidentally initiate a compute intensive process that may block tracking.