Merge pull request #485 from ascott1/Subpart-heading-styles

Initial subpart heading styles
small changes to diff view based on usability tests
re-add history tab highlight with diff view enabled
Merge pull request #486 from ascott1/diff-tab-highlight

re-add history tab highlight with diff view enabled
darken text and icons for interpretation slide downs
merge upstream
Merge pull request #487 from ascott1/usability-test-changes

Usability test changes
resize and tighten padding around subpart headings