use the official android way of doing app install banners

closes: CNVS-32934

see
https://developers.google.com/web/updates/2015/03/increasing-engagement-with-app-install-banners-in-chrome-for-android#native
for how these work.

test plan:
* you have to be serving canvas over https://
* on android, install Chrome, or another browser that
  supports Service Workers and manifest
* Visit the site over two separate days.
* Or, more usefully, enable chrome://flags/#bypass-app-banner-engagement-checks
* it should ask you to install the native canvas android app

Change-Id: I7c5cf8e9ecffd4dc52058bed865f82eaf3a7ea97
Reviewed-on: https://gerrit.instructure.com/93760
Reviewed-by: Simon Williams <simon@instructure.com>
Tested-by: Jenkins
QA-Review: Benjamin Christian Nelson <bcnelson@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>

app/helpers/application_helper.rb

@@ -964,12 +964,17 @@ def custom_dashboard_url
   end
 
   def include_custom_meta_tags
+    output = []
     if @meta_tags.present?
-      @meta_tags.
-        map{ |meta_attrs| tag("meta", meta_attrs) }.
-        join("\n").
-        html_safe
+      output = @meta_tags.map{ |meta_attrs| tag("meta", meta_attrs) }
     end
+
+    # set this if you want android users of your site to be prompted to install an android app
+    # you can see an example of the one that instructure uses in public/web-app-manifest/manifest.json
+    manifest_url = Setting.get('web_app_manifest_url', '')
+    output << tag("link", rel: 'manifest', href: manifest_url) if manifest_url.present?
+
+    output.join("\n").html_safe.presence
   end
 
   # Returns true if the current_path starts with the given value

app/views/layouts/application.html.erb

@@ -37,11 +37,6 @@
 
   css_bundle(:instructure_eportfolio) if @eportfolio_view === true
 
-  if request.user_agent.to_s =~ /Android/i
-    js_bundle :smartbanner
-    css_bundle :smartbanner
-  end
-
   content_for :head, include_common_stylesheets
 
 -%>

bower.json

@@ -18,7 +18,6 @@
     "ic-modal": "0.1.1",
     "ic-droppable": "~0.1.0",
     "ic-sortable": "0.0.2",
-    "jquery.smartbanner": "https://github.com/jasny/jquery.smartbanner.git#8b3810210256ea2760012e18083c43792c2a4146",
     "react": "0.14.8",
     "react-modal": "1.3.0",
     "tinymce": "4.1.9",