[Form] Moved POST_MAX_SIZE validation from FormValidator to request handler

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11729, #11877 
| License       | MIT
| Doc PR        | -
Well, submitting it as an empty form would make thing more complex in case of an edit form, as it would empty all fields rather than keeping the previous values.
I think we should mark the form as submitted, but without submitting the data for children (it could rely on the partial binding though)