Remove HelpUseLocal setting variable. Hide local html skin dir.

We assert that the local html directory is no longer used. Remove machinery
related to its use, notably HelpUseLocal, the code that checks it and the
code that sets and examines special flag URL "__local".
Before actually killing off the local skins/default/html directory, make
viewer_manifest.py rename it but continue packaging it as html.old. If this
doesn't cause a panic, we can proceed with removing it entirely.