#219 Enhance validation of WebAPI submissions

[Ahasuerus]

Currently the Web API only performs high level validation of the XML payload to ensure that it contains a valid XML structure. It does not perform the checks for potential issues that the manual submission creation process performs and it doesn't return any of the warnings that post-submission pages display.

Ideally, we would want the WebAPI to perform the same checks that the manual submission process performs and return the same yellow warning. This is likely to be a non-trivial project due to the way the software is currently written.