show module footer for topics with assignment content tag

fixes CNVS-15922

test plan
- create a graded group discussion
- create a module
- using the api, add the assignment linked to the topic to the
 module (make sure not to add the topic itself, ever)
- add other items to the module so that the module navigation
 footer will have something to navigate too
- navigate to the discussion topic show page
- ensure that the module navigation footer appears at the bottom
 of the page

Change-Id: I74d5bed96a2dec20f5fa66c3a4fd6b4e39e5b1db
Reviewed-on: https://gerrit.instructure.com/42035
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brad Horrocks <bhorrocks@instructure.com>
Reviewed-by: Mark Severson <markse@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Joel Hough <joel@instructure.com>