[Passage] Fix _hasFootnotes() check
[Passage] Fix _hasFootnotes() check

Summary:
_hasFootnotes was using a broken regex to test for whether
there were footnotes for the passage or not. This commit
should fix at least some of the problems with that regex:

 * change `+` to `*`: having zero characters == empty
 * add `^` and `$` to the regex: the whole of the footnote
   content must be spaces for there to be no footnotes, not
   just any character :).

Test Plan:

Try and inspect with web inspector:

 * empty footnote field
 * footnote with text "a footnote"
 * footnote with only a newline

Auditors: john

chirilo/perseus,iamchenxin/perseus,sachgits/perseus,iamchenxin/perseus,abe4mvp/perseus,sachgits/perseus,ariabuckles/perseus,learningequality/perseus,learningequality/perseus,ariabuckles/perseus,ariabuckles/perseus,alexristich/perseus,daukantas/perseus,chirilo/perseus,alexristich/perseus,ariabuckles/perseus,abe4mvp/perseus,daukantas/perseus