Fix unescaped # in SVG data URLs

[patrickhlauke]

Uses a custom string replacement function to ensure that # characters are correctly URL encoded to %23, to avoid Firefox/IE11 choking on the SVG data URLs.

Closes #20956

[patrickhlauke]

@mdo / @cvrebert would appreciate thoughts/review (because of the custom Sass function I had to add)

[jonathanhefner]

Would an easier approach be a SASS function that only escapes hex colors? Such a function would only check the first string character, and would only be applied to the color, instead of the entire URL.

[patrickhlauke]

@mdo @cvrebert as we're getting various duplicate issues filed now, I'm going ahead with merging this...