NEXUS-7575: UA not set for some HTTP exchanges

Like HTTP CONNECT for tunneled proxy. Applied fix
and added UT that checks some cases and verifies UA
is set.
Converts nested panels to subsections
Removes padding from subsections without a top border
Merges subsection changes with master
Merge pull request #823 from sonatype/ui_bugs

Converts additional nested panels to subsections
rename 'sorted-tabpanel' ui to 'light', define tab and bar specifically to control attributes easier
move application of ui: 'light' to surrounding containers away from SortedPanel helper
Add tab scroll images for 'light' ui, pending adjusting these colors to match target style
Use same images for normal and plain
Rebuild baseapp
Tweaks plain button style and more subsection conversion
Fix overlay of baseapp, avoid filtering resources which was corrupting images
optimize regenerate more to re-use the ext dist and generated app
Merge pull request #829 from sonatype/styling_changes

Tweaks plain button style and more subsection conversion
Clean up legacy restlet1x RepositoryURLBuilder, drop methods that exposed forceBaseUrl, use BaseUrlHolder to determine
update test names, drop muck, add terse javadocs
Refine layout and style for nuget tab
Support access to file: schemas when compiling JAXB
Ensure test application is topologically sorted after the test store (previously nothing depended on store, so it could appear anywhere in the sorted graph)
Update mask opacity and colors
Change line in subsection to lighter gainsboro grey