Fixes bug 899174 Update SignatureSummary middleware

* Refine the SQL for SignatureSummary middleware
* Added a connection close() because we aren't using the base class query()
* Adding tests for SignatureSummary* middleware calls
Fixes Bug 820512 - created test_submitter_app
Fixes bug 895928 ignore NULL fields in categories
Fixes 899174 r=@gabithume, @peterbe, @brandonsavage

* PEP8 fixes
* Cleanup of debugging
* Refactor tests to use a dict and a loop
* Remove signatures subselect
* Be sure to truncate all tables in tests
Merge pull request #1377 from selenamarie/bug899174-sigsum-middleware

Fixes bug 899174 update SignatureSummary middleware
Merge pull request #1388 from selenamarie/bug895928-signature-summary-nulls

Fixes bug 895928 ignore NULL fields in categories
Merge pull request #1368 from GabiThume/bug_820512

Fixes Bug 820512 - created test_submitter_app
Fixes 902142 - adds support for partitioning matviews

* Includes a migration that drops foreign keys on
  parent signature_summary tables
* Downgrade restores the FKs
* Also runs backfill_weekly_partitions() for signature_summary tables
Merge pull request #1390 from selenamarie/bug902142-partitioning-for-matviews

Fixes 902142 - adds support for partitioning matviews
Fixes bug 903078 Update to middleware docs
Merge pull request #1393 from selenamarie/bug903078-middleware-docs

Bug903078 middleware docs
fixes bug 902839 - signature summary untested on product_list, r=selenamarie