add better interaction and highlighting on content migration page

fixes CNVS-11644

When selecting content when doing a migration, there are some weird
things that happen with the blue highlighted line. This commit fixes
that so clicking an item will now highlight that item. Also hovering
over an item will have a color and the colors changed.

Test Plan
1. Migrate a course and select the content you want to migrate from
that course.
2. Pull up the Select Content dialog window .
3. Notice that when you hover over the tree items it has a hover state
color.
4. Clicking on an item should change its color to a darker blue.
5. You should be able to use your keyboard to navigate with your
arrows key right after selecting a treeitem.

Change-Id: I2cff94dc322f12eaff499a17287dff4174dcae46
Reviewed-on: https://gerrit.instructure.com/32476
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>
Reviewed-by: Mark Severson <markse@instructure.com>
Product-Review: Sterling Cobb <sterling@instructure.com>