Add Template and string prefetching

This is a hard one to test as it requires a fairly in-depth knowledge and understanding of the way in which Moodle fetches and stores its strings and templates.

Check that the initial prefetch occurs

Since pre-fetching only has an noticeable effect if the resources are not already cached in the browser, it is only active when cachejs is enabled.

1. Disable developer mode (cachejs in particular)
2. Open Moodle to the login page
3. Open developer tools
4. Purge caches via CLI:

   php admin/cli/purge_caches.php

5. In browser developer tools ensure that the "Network" tab is shown
6. Clear the Network tab and force it to show only XHR requests
7. Refresh the login page
   1. Confirm that a short while after the page loaded there were three new requests
8. Look at each request and confirm that you see:
   1. core_load_template
   2. several strings being fetched in a single query
   3. core_output_load_fontawesome
9. Clear the Network tab again
10. Refresh the login page
    1. Confirm that you do not see these requests again
11. Run the following in the "Console" tab of the browser tools

    require(['core/modal_factory'], F => { window.F = F; });

12. Open the "Network" tab and clear the list of network requests
13. Run the following in the browser console

    F.create({title: 'test', body: 'body', type: F.types.SAVE_CANCEL}).then(modal => modal.show());

    1. The modal should open immediately
       Note: if this is tested before MDL-68528 then you will see an extra query because of the lack of normalisation.

If you compare this to master before this change, you will see about 8 queries and the modal is slower to open. I'd recommend slowing the connection from the Network tab of your browser to see this in a really pronounced fashion.

1. CI should continue to pass