WebSubmit: support for elements' custom_level

• Adds support for optionally setting a custom_level ('M' for mandatory or 'O' for optional) for each element through the element description, overriding the default level. Useful for dynamically produced elements. (PR #1741)

Signed-off-by: Nikolaos Kasioumis <nikolaos.kasioumis@cern.ch>
Tested-by: Tibor Simko <tibor.simko@cern.ch>