FEATURE: Allow users to fetch a customized manifest on PWA install

This will allow users installing a Discourse PWA to use their active
theme colors on the generated app. Thanks for @mgiuca for the tip.

Also makes the share_target config explicit to silence Chrome warnings

app/controllers/metadata_controller.rb

@@ -40,6 +40,8 @@ class MetadataController < ApplicationController
       ],
       share_target: {
         action: "/new-topic",
+        method: "GET",
+        enctype: "application/x-www-form-urlencoded",
         params: {
           title: "title",
           text: "body"

app/views/layouts/application.html.erb

@@ -44,7 +44,7 @@
 
     <%= render_google_tag_manager_head_code %>
     <%= render_google_universal_analytics_code %>
-    <link rel="manifest" href="<%= Discourse.base_uri %>/manifest.webmanifest">
+    <link rel="manifest" href="<%= Discourse.base_uri %>/manifest.webmanifest" crossorigin="use-credentials">
 
     <%- if SiteSetting.native_app_install_banner? %>
         <meta name="apple-itunes-app" content="app-id=1173672076">