FIX Don't change scroll position when loading

[GuySartorelli]

The issue was caused by not rendering all of the elements that make up the list of links in the field while loading - which meant they're removed from the DOM, which adjusts the total height of the window.
Once the items are added back to the DOM, the window height adjusts again and therefore the user is not at the same scroll point they were at before the load event occurred.

This is fixed by following the pattern dnadesign/silverstripe-elemental uses - we render the loading component on top of the existing list of items.

The buttons in the link fields can still be accessed by tabbing to them, so I've disabled them when we're loading.

Issue

• Screen jumps up on rerender of MultiLinkField #169

[GuySartorelli]

The act() here and in other tests is needed to avoid a warning:

Warning: An update to LinkField inside a test was not wrapped in act(...).

When testing, code that causes React state updates should be wrapped into act(...):

  act(() => {
    /* fire events that update state */
  });
  /* assert on the output */

This ensures that you're testing the behavior the user would see in the browser. Learn more at https://reactjs.org/link/wrap-tests-with-act

[emteknetnz]

Test locally, works good, could of minor import changes