should not improperly escape imported urls

test plan:
* in html content (such as a wiki page), add a link
 to reference an external tool by url (the tool itself
 doesn't really need to exist, but the link won't work)
 passing in an escaped url that itself has a
 query string referencing an escaped url

e.g.:
if you had a tool configured for http://www.example.com,
you could add a link to

```
<a href="/courses/COURSE_ID/external_tools/retrieve?url=
http%3A%2F%2Fwww.example.com%3Furl%3Dhttp%253A%252F%252F
www.anotherurl.com">Link</a>
```

(replacing COURSE_ID with the course's id)

* copy the course
* the link in the new page should be identical in form
to the old

closes #CNVS-13185

Change-Id: I6bd57e10a7306823061f0d38c8214655a2e197c6
Reviewed-on: https://gerrit.instructure.com/42117
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>