Allow users to add and remove widgets from the admin dashboard.

This is the second half of adding configurability to the admin dashboard. Users
can add widgets to their admin dashboard by clicking on "Add Widgets", which
will open a modal displaying a list of widgets that they can choose from. They
can remove widgets by clicking the X icon on the upper righthand corner of the
widget in question.

Testing Done:
Removed widgets and added them back to make sure that newly added widgets
appeared below the old widgets.

Reviewed at https://reviews.reviewboard.org/r/5622/