Fetch more info about edit files

The type of a file in a change edit is identified in a header on the
getFileInChangeEdit response. This change modifies gr-rest-api to return
the bare response, and upgrades the editor view to parse both the header
identifying the file type and the response body containing the file
content.

Bug: Issue 4437
Change-Id: Icdadeafb4189692cd369c74fe026442787173ef8

polygerrit-ui/app/elements/edit/gr-editor-view/gr-editor-view.html

diff --git a/polygerrit-ui/app/elements/edit/gr-editor-view/gr-editor-view.html b/polygerrit-ui/app/elements/edit/gr-editor-view/gr-editor-view.html
index c6cf7ce..3042e76 100644
--- a/polygerrit-ui/app/elements/edit/gr-editor-view/gr-editor-view.html
+++ b/polygerrit-ui/app/elements/edit/gr-editor-view/gr-editor-view.html
@@ -97,10 +97,9 @@
     </gr-fixed-panel>
     <div class="textareaWrapper">
       <gr-endpoint-decorator name="editor">
-        <gr-endpoint-param name="fileContent" value="[[_newContent]]">
-        </gr-endpoint-param>
-        <gr-endpoint-param name="prefs" value="[[_prefs]]">
-        </gr-endpoint-param>
+        <gr-endpoint-param name="fileContent" value="[[_newContent]]"></gr-endpoint-param>
+        <gr-endpoint-param name="prefs" value="[[_prefs]]"></gr-endpoint-param>
+        <gr-endpoint-param name="fileType" value="[[_type]]"></gr-endpoint-param>
         <textarea value="{{_newContent::input}}" id="file"></textarea>
       </gr-endpoint-decorator>
     </div>