SearchKit - Make loading placeholders configurable

[colemanw]

Overview

This allows the admin to control how many (if any) skeleton results are shown during ajax loading.

It also lightens the colors to make the animation a bit more subtle.

Before

Five placeholders always shown. Animation was reportedly a bit distracting.

After

Colors lightened to be less distracting. Configurable number, or can be disabled completely.

[civibot]

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[clarkac1]

I tested this 6/6/22 and, while it's hard to see the flashing lines because of the speed of the test system, it appears to work just fine. When this is in a nighty build I will download and test on a shared host which is much slower and where the visuals will be much more apparent.

[clarkac1]

Tested again, now I have read the description properly! Downloaded & installed 5.51.alpha1-202206061704 on a shared host because it's slow enough to see the flashing lines. It works as described, although I have to 'Cleanup caches...' in between each change to see the correct result (which is fine).