Ensures that all folders will be listed in wikiSidebar file upload
folder list regardless of the order in which files are processed.

Fixes #363

Before testing commit:
 * Create a folder in a course which comes before "course files" alphabetically.
 * Edit a page. Open the "Files" tab in the sidebar.
 * Click "Upload a new file."
 * Note that the folder you created (for example "class lectures") is not present in the list of folders to which you can upload.

Test plan:
 * Create a folder in a course which comes before "course files" alphabetically.
 * Edit a page. Open the "Files" tab in the sidebar.
 * Click "Upload a new file."
 * Note that the folder you create (for example "class lectures") IS present in the list of folders to which you can upload.
Merge pull request #364 from jswarren/wikiSidebar-file-upload

Returns missing folders to the wikiSidebar file upload folder list