Library XBlocks cannot reference assets named "xblock*"

[bradenmacdonald]

1. Upload an image named xblock-something.png to a Text component in a v2 content library.
2. Reference that image in the HTML, as <img src="/static/xblock-something.png" />
3. The image will not display in the preview, due to this code:

https://github.com/openedx/edx-platform/blob/3e43e33eff7038183c4088fe29209ddcb37ab47e/common/djangoapps/static_replace/__init__.py#L116-L119

[bradenmacdonald]

I suspect a simple fix for this is only ignoring URLs that start with /static/xblock/ rather than /static/xblock

[ChrisChV]

@jmakowski1123 @lizc577 @sdaitzman @marcotuts This is ready for AC testing on the sandbox

[bradenmacdonald]

I originally reported this bug and can confirm this is now working on the sandbox with no issues.