MDL-38455 form: More consistency in section states

- The first section is not-collapsible by default;
- The second section is expanded by default if the form has 2 sections;
- Any section can be set to non-collapsible;
- The collapsed state by setExpanded can override the state in which
  the section was when it was submitted by the user;
- A header containing a required field or one that has an error
  will never be collapsed.