Fix events show view

1. The modals unfortunately are nowhere nearly as responsive
   as Foundation's.  I had to implement someone else's modal
   fixes and tailor those to get something close to what I
   ended up with.  And even at that I had to throw away the
   sign-in modal (below), which turned out to be not so bad
   in the end.  The responsive modal fix uses some hacky JS
   to place the modal at an absolute position and fix it as
   part of that page so as the page scrolls so does the modal.

2. Changed the sign-in modal to a dropdown element in the nav
   bar, so now there's the sign-in form immediately availabe
   in both desktop and mobile platforms; no fussy modal window
   to deal with and less JS to worry about.