Package up the html help

Use the same location that the QtAssistant helpused to go.
Refs #9611
Switch from qtassistant to web browser ...

pointing at a local copy of the documentation.
Refs #9611
Remove qtassistant and wiki tools

No longer required now that the documentation is in the rst files.
Refs #9611
Fix broken category links.

Refs #9611
Use cmake variable to control packaging of offline documentation

It's not built by all platforms on incremental builds so it needs to be
turned off in these situations.
Refs #9611
Move the html package target inside the check for Sphinx.

Refs #9611
Ignore windows html build errors.

Refs #9611
Fix MantidPlot path on the Mac

Refs #9611
Allow for configuration directory on Mac builds too.

Xcode has a similar layout to VS.
Refs #9611
Workaround so that Mac builds can find the properties file.

Refs #9611
Merge remote-tracking branch 'origin/feature/9611_package_html_help'