Add `Style/` prefix for consistency
Update Rubocop version to fix warning

The current locked version of Rubocop doesn't recognize the `Style/`
prefix for the config keys and generates the following warning:
```
Warning: unrecognized cop Style/CollectionMethods found in
/home/su/code/Markus/.rubocop.yml
```

Hence the update.
Improve performance of assigning TAs to students
Add spec for grade entry student mass assignment
Current working code for issue 1636. The Jump To filter now changes its contents according to the selected column in the spreadsheet (i.e. User name, Last Name, or First Name)
Merge pull request #1683 from zhangsu/rubocop_fix

Update Rubocop version to fix warning
Merge pull request #1682 from zhangsu/assign_graders_to_spreadsheet

Improve performance of assigning TAs to students
Fix flaky user name uniqueness in specs

Apparently Faker doesn't generate unique user names in a sequence of
calls and I sometimes get an uniquess violation error because of this.
This can be fixed by using FactoryGirl's sequence.

Tested:
- rspec
Improve unassigning TA performance in spreadsheet

The performance can be improved by doing a bulk delete_all operation. To
achieve this in SQL using an ANSI-compliant way, a list of IDs of the
association model GradeEntryStudentTa is needed. Therefore, the IDs are
both added to the database and the view.
Fix functional tests for new unassign parameters
Add spec for new unassign function
Fix indentation of case ... when statements
Get rid of unnecessary line wraps

These line wraps are no longer needed after the indentation is fixed.
Merge pull request #1685 from zhangsu/user_name_faker

Fix flaky user name uniqueness in specs
Reused existing local variable
add error message to new group with existing name
Merge pull request #1688 from zhangsu/case_indentation_fix

Refactor GradersController#global_actions
Fix failing group creation error test
fix up rspec test message
Refactor assignment model constraints and queries

Constraints are easier to make sense of when grouped by the
attribute that they are constraining.

Many of the query styles that will be deprecated in Rails 4 have
been updated. Though, some cannot be upgraded now as the API does
not yet exist.