fix import/export of folder paths with html entities and unicode

test plan:
* create a folder with unicode characters in the name
* create another folder with an html entity (e.g. "&amp;")
* link to files within those folders inside html content
 in a course (wiki page body, discussion topic message, etc)
* copy or export/import the content
* should preserve the links

fixes #CNVS-13781

Change-Id: I4aa0f0f234834626dad8356440e6f3975e35ffd4
Reviewed-on: https://gerrit.instructure.com/37123
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>