Remove validations and checkboxes from issue/discussion forms

[blakeNaccarato]

Please check for similar discussions first

• I searched and couldn't find an appropriate discussion to participate in instead.

Description

Requiring issue/discussion authors to tick boxes and fill fields ostensibly may affect behavior in such a way that reduces workload for maintainers, or at least that's the idea. But the additional boilerplate of checkboxes and repetitive field information creates visual noise in issue posts.

Also, required text entry fields interact poorly with Ctrl+Enter posting, where pressing it will clear the field instead of posting the issue/discussion. So, no fields should be required, and the "I searched for similar" tickbox should be converted to a non-submitted Markdown admonition.

[blakeNaccarato]

Also demote headings to level 4.

django/django#18499