Implement collections API filter and fallback system properly (bug 915803)

- Filters no longer automatically include NULL values
- Fallback no longer drops filters, instead replace the corresponding
  value with NULL
Disconnect signals until last save in addon_factory
add rejected apps to list_packaged_apps management command
fix verify tests by patching right place
Merge pull request #1134 from diox/collections-fallback-null

Implement collections API filter and fallback system properly (bug 915803)
refactor and fix queue sorting for name fields and pending queue (bug 890177)
Merge pull request #1123 from ngokevin/revqueuesort

refactor and fix queue sorting for name fields and pending queue (bug 890177)
add removeslash middleware back
Strip extraneous whitespace in API JSON responses (bug 914876)
Merge pull request #1138 from chuckharmston/914876-json_succinct

Strip extraneous whitespace in API JSON responses (bug 914876)
Now that fallback system is fixed, use it for every collection type (bug 915803)

Also fix tests.