Morph API::Helpers into controller Filters

A better place for shared routines between controllers, now sitting in
app/controllers/filters.

Closes CNVS-10071

TEST PLAN
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It's all renaming changes so if the specs pass things should be ok.

Change-Id: I6c3d155a487d47e6769216d6e61fe33d438a0782
Reviewed-on: https://gerrit.instructure.com/29511
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
QA-Review: Myller de Araujo <myller@instructure.com>
Product-Review: Ahmad Amireh <ahmad@instructure.com>