Return file's media_entry_id in response

fixes MAT-959
flag=none

Test Plan 1: Backwards compatibility
- Get the changes in this commit locally in your RCS
- In your local canvas, run a recent version of master
- In canvas, go to an RCE
- From tools menu: Insert > Document > Course documents
- In the dropdown that says "Document", change to "All"
- Select a file from either "Course files" or "My files"
VERIFY:
- file is properly embedded in the RCE.

Test Plan 2: Together with corresponding changes in the canvas-lms repo
- https://gerrit.instructure.com/c/canvas-lms/+/305518
- follow the test plan outlined in that commit

Change-Id: I4aeaf098c8192d9664f7ef89cfa89466d328a5c0
Reviewed-on: https://gerrit.instructure.com/c/canvas-rce-api/+/305519
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Ed Schiebel <eschiebel@instructure.com>
QA-Review: Ed Schiebel <eschiebel@instructure.com>
Product-Review: Joe Hernandez <joe.hernandez@instructure.com>

Author: Joe Hernandez

app/api/files.js

@@ -38,7 +38,8 @@ function canvasResponseHandler(request, response, canvasResponse) {
           folderId: file.folder_id,
           iframeUrl: file.embedded_iframe_url,
           thumbnailUrl: file.thumbnail_url || file.url,
-          category: file.category
+          category: file.category,
+          mediaEntryId: file.media_entry_id
         };
       }),
       bookmark: packageBookmark(request, canvasResponse.bookmark)

test/service/api/files.test.js

@@ -115,7 +115,8 @@ describe("Files API", () => {
           folder_id: 1,
           embedded_iframe_url: "https://canvas.com/foo/bar",
           thumbnail_url: "https://canvas.com/foo/bar/thumbnail",
-          category: "foo"
+          category: "foo",
+          media_entry_id: "bar"
         };
       });
 
@@ -139,7 +140,8 @@ describe("Files API", () => {
             folderId: 1,
             iframeUrl: "https://canvas.com/foo/bar",
             thumbnailUrl: "someurl",
-            category: "foo"
+            category: "foo",
+            mediaEntryId: "bar"
           });
         });
       });
@@ -156,6 +158,7 @@ describe("Files API", () => {
       });
 
       it("files have correctly tranformed properties", () => {
+        file.media_entry_id = null;
         canvasResponse.body = [file];
         canvasResponse.statusCode = 200;
         canvasResponseHandler(request, response, canvasResponse);
@@ -170,7 +173,8 @@ describe("Files API", () => {
           folderId: 1,
           iframeUrl: "https://canvas.com/foo/bar",
           thumbnailUrl: "https://canvas.com/foo/bar/thumbnail",
-          category: "foo"
+          category: "foo",
+          mediaEntryId: null
         });
       });