Pages and Content Blocks: TinyMCE's icons for its WYSIWYG are blank

[DiemBTran]

Descriptive summary

TinyMCE's icons for its WYSIWYG on Pages and Content Blocks are blank. The buttons are still operable, but the icon images no longer show. I have seen this on at least 2 apps: British Libraries and WUSTL - NNP

Expected behavior

The buttons should work and have their appropriate icons

Actual behavior

Steps to reproduce the behavior

1. As an admin, go to dashboard > settings > pages / content blocks
2. Press buttons on WYSIWYG and add text to see that the buttons are still operable
3. Observe, however, that the icon images no longer show, which makes the WYSIWYG less usable

Related work

Link to related tickets or prior related work here.

[summer-cook]

this was recently fixed in Adventist: https://github.com/scientist-softserv/adventist-dl