Add separate Post check for is_valid_body_field

[Neshura87]

Implements #3262

[Neshura87]

Woodpecker reran without the commit change, is there any way to rerun the validation?

Edit: Adding another commit by adding an empty line worked

[mpusch88]

It seems like this may not be working. Attempting to post 22417 chars in markdown format results in an 'invalid_body_field' error, and an infinite loading state.

Note that the preview loads fine - I guess it's also possible that there is an illegal character or something.

[Neriderc]

For anyone arriving here wondering why they can only post 10k characters - well I don't have an answer for you, but know that if you post it first, then you can edit it and it allows the 50k in the edit.