Issue 4700 - Allow specifying devbuilds update URL in manifest.json for WebExtensions builds

templates/manifest.json.tmpl

 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 {
   "manifest_version": 2,
   {%- if releaseBuild %}
   "name": "__MSG_name__",
   {%- else %}
   "name": "__MSG_name_devbuild__",
@@ skipping 20 matching lines @@
     {
       "id": {{metadata.get('general', 'id')|json}}
       {%- if metadata.has_section('compat') and metadata.has_option('compat', 'gecko') %}
       ,
       "strict_min_version": {{metadata.get('compat', 'gecko')|json}}
       {%- endif %}
     }
   },
   {%- endif %}
 
+  {%- set updateURLSetting = 'updateURL' if releaseBuild else 'devbuildUpdateURL' %}
+  {%- if metadata.has_option('general', updateURLSetting) %}
+  "update_url": {{metadata.get('general', updateURLSetting)|json}},
+  {%- endif %}
+
   {%- if pageAction %}
   "page_action": {
     "default_icon": {{pageAction.icon|json}},
     {%- if pageAction.popup %}
     "default_popup": {{pageAction.popup|json}},
     {%- endif %}
     "default_title": "__MSG_name__"
   },
   {%- endif %}
 
@@ skipping 50 matching lines @@
   {%- endif %}
 
   {%- if metadata.has_option('general', 'managedStorageSchema') %}
   "storage": {
     "managed_schema": {{metadata.get('general', 'managedStorageSchema')|json}}
   },
   {%- endif %}
 
   "_dummy": false
 }