Add a credentials option to the asset manager config to allow overriding the default 'include' option

docs/modules/Assets.md

@@ -63,6 +63,9 @@ Below the list of currently available options
   // Custom parameters to pass with the upload request, eg. csrf token
   params: {},
 
+  // The credentials setting for the upload request, eg. 'include', 'omit'
+  credentials: 'include',
+
   // If true, tries to add automatically uploaded assets.
   // To make it work the server should respond with a JSON containing assets
   // in a data key, eg:

src/asset_manager/config/config.js

@@ -29,6 +29,9 @@ module.exports = {
   // Custom parameters to pass with the upload request, eg. csrf token
   params: {},
 
+  // The credentials setting for the upload request, eg. 'include', 'omit'
+  credentials: 'include',
+
   // If true, tries to add automatically uploaded assets.
   // To make it work the server should respond with a JSON containing assets
   // in a data key, eg:

src/asset_manager/view/FileUploader.js

@@ -122,7 +122,7 @@ module.exports = Backbone.View.extend(
         this.onUploadStart();
         return fetch(url, {
           method: 'post',
-          credentials: 'include',
+          credentials: config.credentials || 'include',
           headers,
           body
         })