Make preview charset fix

[Mirobit]

Since the incremental build (#1643) update "make preview" leads to the following error:
Conversion error: Jekyll::Converters::Scss encountered an error while converting 'css/main.scss': Invalid US-ASCII character "\xE2" on line 2427 jekyll 3.0.1 | Error: Invalid US-ASCII character "\xE2" on line 2427
I don't know if this is the best solution but it fixes the error.

[harding]

Concept ACK and this should be safe for immediate merge since it's just changing the non-production preview target.. I'm testing now---I should have results in about 30 minutes.

Sorry, I didn't attempt to test incremental building outside my usual UTF-enabled environment. I think this solution is ideal given that the regular build target does exactly the same thing, and that's what we use in production.

[harding]

Tested ACK now. This change renders the CSS files (and all other files) in the preview target as in the production build target.

[wbnns]

@Mirobit Nice catch! @harding Thanks for double-checking.

Merged. 👍