Allow the header bar customization

This commit also provide a way to escape the Shorcuts page on
Sandstorm.

client/components/main/keyboardShortcuts.jade

@@ -0,0 +1,19 @@
+template(name="shortcutsHeaderBar")
+  h1
+    a.back-btn(href="{{pathFor 'home'}}")
+      i.fa.fa-chevron-left
+    | {{_ 'keyboard-shortcuts'}}
+
+template(name="shortcutsModalTitle")
+  h2
+    i.fa.fa-keyboard-o
+    | {{_ 'keyboard-shortcuts'}}
+
+template(name="keyboardShortcuts")
+  .wrapper.shortcuts-list
+    each mapping
+      .shortcuts-list-item
+        .shortcuts-list-item-keys
+          each keys
+            kbd= this
+        .shortcuts-list-item-action {{_ action}}