Add option to make storage persistent to Offline page.

Fixes #703

assets/javascripts/templates/error_tmpl.coffee

@@ -51,6 +51,14 @@ app.templates.offlineError = (reason, exception) ->
 
   error 'Offline mode is unavailable.', reason
 
+app.templates.persistenceError = (exception) ->
+  reason = if exception
+    """<code class="_label">#{exception.name}: #{exception.message}</code>"""
+  else
+    """Bookmark this site and try again."""
+
+  error 'Persistence request denied by browser.', reason
+
 app.templates.unsupportedBrowser = """
   <div class="_fail">
     <h1 class="_fail-title">Your browser is unsupported, sorry.</h1>

assets/javascripts/templates/pages/offline_tmpl.coffee

@@ -1,4 +1,4 @@
-app.templates.offlinePage = (docs) -> """
+app.templates.offlinePage = (docs, hasPersistence, isPersistent) -> """
   <h1 class="_lined-heading">Offline Documentation</h1>
 
   <div class="_docs-tools">
@@ -21,7 +21,10 @@ app.templates.offlinePage = (docs) -> """
       #{docs}
     </table>
   </div>
-  <p class="_note"><strong>Note:</strong> your browser may delete DevDocs's offline data if your computer is running low on disk space and you haven't used the app in a while. Load this page before going offline to make sure the data is still there.
+  </div>
+  <div id="_offline-persistence-note">
+    #{offlinePersistenceNote(hasPersistence, isPersistent)}
+  </div>
   <h2 class="_block-heading">Questions & Answers</h2>
   <dl>
     <dt>How does this work?
@@ -78,3 +81,19 @@ app.templates.offlineDoc = (doc, status) ->
     """
 
   html + '</tr>'
+
+offlinePersistenceNote = (hasPersistence, isPersistent) ->
+  if isPersistent
+    return ''
+
+  html = """
+    <p class="_note">
+      <strong>Note:</strong> your browser may delete DevDocs's offline data if your computer is running low on disk space and you haven't used the app in a while.
+  """
+
+  if hasPersistence
+    html += ' <button type="button" class ="_btn-link _bold" data-enable-persistence>Enable persistent storage</button>.'
+  else
+    html += ' Load this page before going offline to make sure the data is still there.'
+
+  return html

assets/javascripts/views/content/offline_page.coffee

@@ -20,10 +20,11 @@ class app.views.OfflinePage extends app.View
       if statuses is false
         @html @tmpl('offlineError', app.db.reason, app.db.error)
       else
-        html = ''
-        html += @renderDoc(doc, statuses[doc.slug]) for doc in app.docs.all()
-        @html @tmpl('offlinePage', html)
-        @refreshLinks()
+        @checkPersistence (hasPersistence, isPersisted) =>
+          html = ''
+          html += @renderDoc(doc, statuses[doc.slug]) for doc in app.docs.all()
+          @html @tmpl('offlinePage', html, hasPersistence, isPersisted)
+          @refreshLinks()
       return
     return
 
@@ -60,6 +61,8 @@ class app.views.OfflinePage extends app.View
       return unless action isnt 'uninstall' or window.confirm('Uninstall all docs?')
       app.db.migrate()
       $.click(el) for el in @findAll("[data-action='#{action}']")
+    else if el.hasAttribute('data-enable-persistence')
+      @requestPersistence()
     return
 
   onInstallSuccess: (doc) ->
@@ -90,3 +93,24 @@ class app.views.OfflinePage extends app.View
     if event.target.name is 'autoUpdate'
       app.settings.set 'manualUpdate', !event.target.checked
     return
+
+  checkPersistence: (callback) ->
+    if navigator.storage and navigator.storage.persisted
+      navigator.storage.persisted().then((persisted) ->
+          callback true, persisted
+        ).catch ->
+          callback false, persisted
+    else
+      callback false, false
+
+  requestPersistence: ->
+    navigator.storage.persist().then((success) =>
+      @onPersistenceRequestCompleted success
+    ).catch (exception) =>
+      @onPersistenceRequestCompleted false, exception
+
+  onPersistenceRequestCompleted: (success, exception) ->
+    if success
+      @render()
+    else
+      @html @tmpl('persistenceError', exception)