forms: add validation of layout and stylesheet
forms: add validation of layout and stylesheet

We have received several rendering requests with layout and stylesheet
empty. We believe that people might have written scripts against the
previous version of MapOSMatic, and so they didn't fill those
fields. Therefore, we add some form-level validation logic to reject
jobs that have layout and/or stylesheet empty.

Signed-off-by: Thomas Petazzoni <a5d87d2bd7c424ee5cbdb4e978635bf653d5a005@enix.org>

hholzgra/maposmatic,maposmatic/maposmatic,hholzgra/maposmatic,hholzgra/maposmatic,maposmatic/maposmatic,hholzgra/maposmatic,maposmatic/maposmatic,hholzgra/maposmatic,maposmatic/maposmatic