fix: avoid unnecessary rerenders

[spereirag]

Summary

My team currently has a fork of decap-cms with several custom plugins, some of them considerably costly in terms of render time. We noticed the render time became unbearably slow when loading a moderately long list (~15 elements) of one of said plugins. After a bit of debugging, we noticed a huge amount of component rerenders while the assets were loading, and the plugin wasn't even visible. The problem is fixed when using the existing prop isLoadingAsset to avoid updating the component while the assets are loading. This PR implements that fix.

Test plan

yarn test runs successfully

Test Suites: 1 skipped, 79 passed, 79 of 80 total
Tests:       652 skipped, 989 passed, 1641 total
Snapshots:   110 passed, 110 total
Time:        79.975 s
Ran all test suites.

Checklist

Please add a x inside each checkbox:

• I have read the contribution guidelines.

[netlify]

‼️ Deploy request for cms-demo rejected.

Name	Link
🔨 Latest commit	5b8763b

[netlify]

✅ Deploy Preview for decap-www canceled.

Name	Link
🔨 Latest commit	4bb2170
🔍 Latest deploy log	https://app.netlify.com/sites/decap-www/deploys/6581b5979795cd0007ac3ac1