Update validation masthead pattern

[samuelhwilliams]

Summary

GOV.UK validation masthead patterns use h2 rather than h1 for their
validation mastheads which appear at the top of the page indicating some
errors during form submission. There are also accessibility and general
HTML-compliance issues with having more than one h1 on the page. Best
practice and guidance therefore indicates we should change our template
to using an h2 heading.

Finally, we include the validation.html in the base page layout for
the frontend toolkit, ensuring that all pages will correctly show
validation when errors are passed into the rendering template as
errors. This means that individual pages no longer need to include the
boilerplate code for including validation and it's available out of the
box.

Ticket

https://trello.com/c/RSYEM3FL/448

[katstevens]

I think we should make it clear that errors is now a nested dict rather than a list of dicts. This will potentially affect the ordering of validation messages (which was an issue for the password change form errors in the User FE). Maybe add a New code sample 3 with an example of all the stuff that's needed for the toolkit template:

ordered_form_errors = OrderedDict([
    (`key1`, {'question': 'First question', 'input_name': 'new_password', 'message': 'Not long enough'}),
    (`key2`, {'question': 'Second question, 'input_name': 'repeat_new_password', 'message': 'Does not match'}),
])
return render_template('blah.html', errors=ordered_form_errors)

[katstevens]

Ah - just read through the utils PR 😹 Maybe an OrderedDict example isn't needed but I think it's still worth being clear what format we expect to be returned from get_errors_from_wtform() (and emphasise here that we should be using it where possible).

[samuelhwilliams]

Good idea - I'll clarify this documentation a tad. 👍

[samuelhwilliams]

Docs updated.

[katstevens]

Good stuff 👍