Made sort_by and pagination fixes

Fixed sort_by on user_name and last_name
Added alpha pagination for sorting by user_name
Moved alpha pagination to helper and added alpha pagination to first_name sorting
Per_page in grades view being gotten from cookies instead of always reset to 15
Fixed sort_by with ascending or descending
Refactored alpha_paginate method
Updated alpha pagination tests

Moved tests to a new helper test file. Added tests for sorting by
user_name and first_name.
Pagination/sorting bug fixes. Added sorting by section.

Sorting by section returns students with no section ('include' does
outer join, 'join' does inner join)

Sorting by section orders students within a section by increasing
user_name
Fix for issue-1294