FEATURE: Enable the Web Share Target API

This will allow a Discourse instance that was installed[1] to receive share events.

See https://wicg.github.io/web-share-target/ for the spec.

1: https://developers.google.com/web/fundamentals/app-install-banners/

app/controllers/metadata_controller.rb

@@ -21,6 +21,9 @@ class MetadataController < ApplicationController
       display: 'standalone',
       orientation: 'natural',
       start_url: "#{Discourse.base_uri}/",
+      share_target: {
+        url_template: 'new-topic?title={title}&body={text}%0A%0A{url}'
+      },
       background_color: "##{ColorScheme.hex_for_name('secondary')}",
       theme_color: "##{ColorScheme.hex_for_name('header_background')}",
       icons: [