Concatenated all JS into a single file (#624)

no issue

- moved large inline JS from templates into separate JS files
  - floating header
  - gallery card support
- use `gulp-concat` to concatenate all JS files into a single `built/casper.js` file
- reduces external JS file requests from 3 (jquery, infinite-scroll.js, jquery.fitvids.js) down to 2 (jquery, casper.js) and reduces page size by removing repeated inline code

yarn.lock

@@ -674,6 +674,13 @@ concat-stream@^1.5.2, concat-stream@^1.6.0:
     readable-stream "^2.2.2"
     typedarray "^0.0.6"
 
+concat-with-sourcemaps@^1.0.0:
+  version "1.1.0"
+  resolved "https://registry.yarnpkg.com/concat-with-sourcemaps/-/concat-with-sourcemaps-1.1.0.tgz#d4ea93f05ae25790951b99e7b3b09e3908a4082e"
+  integrity sha512-4gEjHJFT9e+2W/77h/DS5SGUgwDaOwprX8L/gl5+3ixnzkVJJsZWDSelmN3Oilw3LNDZjZV0yqH1hLG3k6nghg==
+  dependencies:
+    source-map "^0.6.1"
+
 config-chain@~1.1.5:
   version "1.1.12"
   resolved "https://registry.yarnpkg.com/config-chain/-/config-chain-1.1.12.tgz#0fde8d091200eb5e808caf25fe618c02f48e4efa"
@@ -1672,6 +1679,15 @@ gulp-cli@^2.2.0:
     v8flags "^3.0.1"
     yargs "^7.1.0"
 
+gulp-concat@^2.6.1:
+  version "2.6.1"
+  resolved "https://registry.yarnpkg.com/gulp-concat/-/gulp-concat-2.6.1.tgz#633d16c95d88504628ad02665663cee5a4793353"
+  integrity sha1-Yz0WyV2IUEYorQJmVmPO5aR5M1M=
+  dependencies:
+    concat-with-sourcemaps "^1.0.0"
+    through2 "^2.0.0"
+    vinyl "^2.0.0"
+
 gulp-livereload@4.0.2:
   version "4.0.2"
   resolved "https://registry.yarnpkg.com/gulp-livereload/-/gulp-livereload-4.0.2.tgz#fc8a75c7511cd65afd2202cbcdc8bb0f8dde377b"