Don't user issuable_sort cookie for epics collection

Merged Jarka Košanová requested to merge 4114-epic-list-fix into master

What does this MR do?

It fixes the error on epic list page by rewriting the IssuableCollections method that sets the sort param from a cookie.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

When a user sorted eg. issues by labels the sorting was stored in the cookie. This cookie was later used again by IssuableCollections which we use for epics too. But we don't support custom sorting for epics + not everything we support on issues/MRs is supported on epics (eg. labels are not yet supported)

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

What are the relevant issue numbers?

Closes #4114 (closed)