Sorting preference functionality has been extracted
from `IssuableCollections` to a new `SortingPreference`
concern in order to reuse this functionality in projects
(and groups in the future).
This mistake seems to have always been there, but it only resulted in
errors on the `/explore*.json` since they were the one that _actually_
relied on the local variables.
Updates specs to use new rails5 format.
The old format:
`get :show, { some: params }, { some: headers }`
The new format:
`get :show, params: { some: params }, headers: { some: headers }`
