extensionSystem: Update immediately after major upgrades

Currently we periodically check for updated extensions, prepare an update and perform it at the next login.

This is largely due to the fact that once an extension has been loaded, its code is cached and reloading it would only make it appear as updated, while in reality still running the old code.

Of course this only applies once we have loaded extensions.

Before that, it's possible to download and install updates, and only then initialize extensions with their latest version.

The trade-off is that network requests, data download and extraction may introduce a significant delay before extensions are enabled. Most extensions modify the UI one way or another, so that delay would likely be noticeable by the user.

Assuming that users are usually happy enough with the current extension version, that trade-off doesn't seem worthwhile.

However there is an exception: After a major version update, extensions are likely disabled as out-of-date, or at least more likely to break (when the version check is disabled).

In that case delaying extension initialization to download and install updates looks like the better trade-off, so do that.