Implemented moving group to a step

[hirokiterashima]

Changes

• Teacher can move group to step via the manage groups view
• Cleaned up code

Test

• First checkout and pull from the "manage-groups" branch in SCORE-API, to make sure that you have the updated backend code for moving group to a step

  SCORE-API $ git checkout manage-groups
  SCORE-API $ git pull

• In the manage groups view, click on an existing group to edit. Icon to move the group should appear at the top right
• Can move group to a step- all the members in the group who are logged in will be moved to the step
• Can change membership (add/delete members), and move the group to step. This should only move the updated members in the group to the step.

closes #4