Make the consent screens (almost completely) translatable

One exception is the wording for the privacy policy/TOS, because it's
annoying to do with the conditionals.

templates/pages/device_link.html

@@ -23,8 +23,8 @@ limitations under the License.
     </div>
 
     <div class="header">
-      <h1 class="title">Link a device</h1>
-      <p class="text">Enter the code displayed on your device</p>
+      <h1 class="title">{{ _("mas.device_code_link.headline") }}</h1>
+      <p class="text">{{ _("mas.device_code_link.description") }}</p>
     </div>
   </header>