use UTF-8-aware excerpt in feed widgets

[nbachiyski]

When showing excertps the feed widgets calls substr() directly, which works on byte level and not on character level. Thus UTF-8 characters are split into two and shown as diamonds in browsers.

We have wp_html_escerpt() which does the trick.

[ryan]

(In [9791]) Use wp_html_excerpt() instead of substr in rss widget. Props nbachiyski. fixes #8278