Refactor some more models

Grouping attribute constraints, and updating some queries for Rails 4.
fix grader in criteria checkbox bug
scope, props refactor for students_table
scope, props refactor in groups manager
basic refactor for submissions table
move stuff to default props in summaries table
add some comments to graders manager
make session store point to config settings
ok, now fixed selected all criteria
Merge branch 'finders6' of https://github.com/ferrous26/Markus into finders6
Hound complaints
Merge pull request #1695 from oneohtrix/react-criteria-checkbox-patch

fix grader in criteria checkbox bug. Fixes #1680
Merge pull request #1699 from oneohtrix/session-patch

make session store point to config settings
Merge pull request #1697 from oneohtrix/react-refactor

React refactor
Fix method name in markus:load:results

The method has been renamed to `update_results_stats`, but the invocation in the task is not updated.
Merge pull request #1708 from zhangsu/patch-1

Fix method name in markus:load:results