Help menu : Tidy up and fix broken items.
Help menu : Tidy up and fix broken items.

- Fix OSL link. It seems that the OSL project changed where they install their docs.
- Deactivate items if file cannot be found for any reason (perhaps a custom build with an external OSL).
- Remove unused code.
- Link to new HTML docs.
- Rename "Developer Discussion" to a more inclusive "Mailing List"
- Prefer `functools.partial` to `IECore.curry`

johnhaddon/gaffer,GafferHQ/gaffer,hradec/gaffer,appleseedhq/gaffer,chippey/gaffer,andrewkaufman/gaffer,andrewkaufman/gaffer,appleseedhq/gaffer,chippey/gaffer,ImageEngine/gaffer,lucienfostier/gaffer,andrewkaufman/gaffer,andrewkaufman/gaffer,ImageEngine/gaffer,boberfly/gaffer,appleseedhq/gaffer,chippey/gaffer,lucienfostier/gaffer,johnhaddon/gaffer,lucienfostier/gaffer,andrewkaufman/gaffer,hradec/gaffer,johnhaddon/gaffer,ImageEngine/gaffer,hradec/gaffer,johnhaddon/gaffer,boberfly/gaffer,boberfly/gaffer,GafferHQ/gaffer,appleseedhq/gaffer,hradec/gaffer,hradec/gaffer,ivanimanishi/gaffer,lucienfostier/gaffer,boberfly/gaffer,ImageEngine/gaffer,johnhaddon/gaffer,ivanimanishi/gaffer,GafferHQ/gaffer,ivanimanishi/gaffer,chippey/gaffer,GafferHQ/gaffer,ivanimanishi/gaffer,GafferHQ/gaffer