Make PARAVIEW_ENABLE_WEB a dependent option

Make PARAVIEW_ENABLE_WEB a dependent option so its only
available if PARAVIEW_ENABLE_PYTHON is enabled.

Change-Id: I775c604a03afcfd5da6e34369a4bc98911c8859c
Merge topic '0013949-web-enable-dependent-option' into next

b89b93e Make PARAVIEW_ENABLE_WEB a dependent option