[pkg-fgfs-crew] Preparing for flightgear 3.6

[Rebecca N. Palmer]

#793517 turned out to be not ours (and is now fixed), but for the new 
flightgear-data dependencies I only got as far as making this list (I 
have _not_ tested whether it actually works with these replacements):

flot - already replaced
jquery - already replaced
jquery-ui-themes-1.11.2 - lots of libjs-jquery-ui-theme-* (appears to be 
user-selectable, so does use them all)
knockout - not in Debian
knockout-jqueryui - *maybe* libjs-jquery-ui
leaflet-0.7.3 - already replaced
pagedown - not in Debian
require - libjs-requirejs + libjs-requirejs-text
sprintf - already in missing-sources (needs lintian override, #787592)
sammy-latest.min.js - not in Debian