Feature/import redirect error

Fix for [PR 532](https://github.com/reciprocity/ggrc-core/pull/532) on [story 55437112](https://www.pivotaltracker.com/story/show/55437112).  Turns out there were two problems:

1) The mustache template has separate sections for linking the sections vs. clauses import page, the former of which was missing the query string.  This is corrected in the PR.

2) In cases where a redirect url (the `return_to` param) is not passed along, it only has `None` as the redirect target, which was the proximate cause of this error.

We need to handle the case where no such parameter is passed along.  In that case, this PR sets the view function to default to the redirect location (in the `return_to` var) we were using before.