Block previews causing crashes

[brian-smith-tcril]

I have been able to reproduce this issue somewhat consistently by:

1. Creating a library with 20+ blocks (it seems any type works)
2. Enabling previews
3. Scrolling up/down while the previews are loading

Sometimes refreshing the page with previews enabled will lead to the crash. Sometimes things seem to work without crashing. In the browser console I encounter a wall of errors:

One idea I had to address this without diving into why loading 20+ copies of the same javascript in iframes causes crashes is to not load 20+ copies of the same javascript every time. One way this could be accomplished by switching from a global "show previews" toggle to a per-block "show preview" toggle.

[jristau1984]

This issue has been brought into the 2U Library Authoring epic with this internal ticket: https://2u-internal.atlassian.net/browse/TNL-10591

[rayzhou-bit]

I could not get this issue to reproduce on hosted devstack or stage.