Improve the UI of the Add Columns modal

[jernestmyers]

Relates to #246

During the implementation of the new CheckboxTree, the following UI improvements will be made to this component:

• remove the "Update Columns" button from the top of the modal and the treeLinks from the bottom of the modal
• ensure that the "columns selected" message, the treeLinks, and the search box remain fixed at the top of the modal
• ensure that the "Update Columns" button remains in view at the bottom of the modal
• only scroll the treeSection (relates to prior 2 tasks)
• unify the tooltips used in the treeSection and the "Update Columns" button

[jernestmyers]

Closed by #249 although the last task regarding the tooltips has been kicked to a new issue. The tooltips in the treeSection have been temporarily disabled due to performance reasons.