Remove whitespace for effective at-mentions

If the display name contained whitespace, the function would keep the
whitespace in the at-mention.  According to observed behavior on the
website, this is not correct.

This commit removes all whitespace from the returned display name in the
appropriate function.
Streamline replying to comments

Append a single space after the at-mention in comment replies.
Merge pull request #98 from vermiculus/fix-at-mention-string

Fix at-mention string