REV BUG: revert usage of paginator module

submission/models.py uses paginator class to find next, previous submissions, revision.
This implementation expects no discontinuity in `pk` values of the QuerySet passed to paginator.

This reverts commit 35f3395c2eaf149e4d2392606a24ac237a753c26.
Merge pull request #176 from ksurya/fix-pag

BUG: revert usage of paginator module