Fix large gap between results in Instant Results when there are few results

[JakePT]

Description of the Change

Fixes an issue where a large gap appeared between results when few results were being displayed (the exact amount depended on display size). The fix replaces the use of CSS grid with using the gap property with a flex object, which is now supported by the last 2 major versions of all major browsers.

This also includes a tweak to how the Next and Previous buttons are hidden when unavailable. Previously they were simply not rendered, but now they are rendered and hidden with visibility: hidden. This allows them to occupy space which will prevent the "Page X of Y" from shifting around as the buttons are rendered and un-rendered.

Closes #3173

How to test the Change

1. Activate Instant Results.
2. Perform a search or filter such that the number of returned results does not require scrolling, and where a large gap would be expected after the last result.
3. Check that the results are no inappropriately spread out vertically. Instead there should be small gaps between results and a large gap between the last result and the "Page X of Y" text.

Changelog Entry

Fixed - An layout issue in Instant Results that would occur with small result sets.

Credits

Props @JakePT

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.