Fix assets by using relative URLs to point ot them.

[tvst]

fixes #1725

[nthmost]

Confirmed that this commit effectively reverts a problematic change.

[kantuni]

@tvst This will break /assets endpoint that custom components use:

streamlit/lib/streamlit/file_util.py

Line 124 in d7d9c64

def get_assets_dir():

[tvst]

@kantuni is there a test for this? If not, how do I test?
(Also, we should add a test here)

[tvst]

Henrikh responded:

I don’t think we have tests for the assets endpoint

It’s not used currently, so feel free to merge it (as it’s a release blocker)

[tconkling]

This will be used by the Streamlit.loadCustomCSS() function we'll be adding to the component template. But we punted on this for the launch, because there were some layout issues. There aren't any tests for it yet (but - @kantuni, could we easily write one that just tries to access /assets/streamlit.css?)