Improve responsive design on mobile devices (vuejs#428)

* Fix menu button on tablets

* Hide menu, collapse header on tablets

* Center content on mobile devices

Author: lin-lu

themes/vue/source/css/_sidebar.styl

@@ -58,7 +58,7 @@
         display none
         padding-left 0
 
-@media screen and (max-width: 720px)
+@media screen and (max-width: 900px)
     .sidebar
         position fixed
         z-index 8

themes/vue/source/css/page.styl

@@ -142,7 +142,7 @@
             bottom 12px
             left 140px
 
-@media screen and (max-width: 720px)
+@media screen and (max-width: 900px)
     #ad
         width 100%
     body
@@ -164,7 +164,7 @@
         padding 1.2em 1em
     .content
         &.with-sidebar
-            margin-left 0
+            margin auto
     .footer
         margin-left 0
         text-align center