When updating a project submission with invalid link, other submitted fields are reset

[alexeygrigorev]

Steps to reproduce

• create a project submission
• edit it
• replace the project url with invalid link
• add more learning in public links
• submit
• the updated learning in public links are gone

The test test_submission_exist_post_with_error in test_project_view.py reproduces it (currently commented)

A fix would require a major redesign of the project view. One of the potential solutions would be using a django form and validations there, e.g. like in the enrollement vide