Migrate assets to use Webpacker

Gemfile.lock

@@ -277,6 +277,8 @@ GEM
     rack (2.0.5)
     rack-protection (2.0.3)
       rack
+    rack-proxy (0.6.4)
+      rack
     rack-test (1.0.0)
       rack (>= 1.0, < 3)
     radix62 (1.0.1)
@@ -419,6 +421,10 @@ GEM
       addressable (>= 2.3.6)
       crack (>= 0.3.2)
       hashdiff
+    webpacker (4.0.0.pre.pre.2)
+      activesupport (>= 4.2)
+      rack-proxy (>= 0.6.1)
+      railties (>= 4.2)
     webrobots (0.1.2)
     websocket-driver (0.7.0)
       websocket-extensions (>= 0.1.0)
@@ -507,6 +513,7 @@ DEPENDENCIES
   unicorn
   unicorn-worker-killer
   webmock
+  webpacker (>= 4.0.x)
   whenever
 
 BUNDLED WITH