Related Posts: Bring UI in line with WordPress.com version

Some niceties / UI tweaks for related posts:

* Limit excerpts to 5 lines max (resolves #1249)
* Use UTF-8 ellipsis to prevent double encoding issue (#1282)
* Make entire excerpt clickable
Merge pull request #1353 from xyu/relatedpost-tweak-ui

Related Posts: Bring UI in line with WordPress.com version