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"