Before, custom classes passed in the `label` hash would override any existing
classes.

For example:

```erb
<%= form_with model: @model do |f| %>
  <%= f.govuk_text_area :details, label: { text: "Some details", class: "special-class" } %>
<% end %>
```

would output:

```html
<!-- form markup --!>

<label for="model-details-field" class="special-class">
  Some details
</label>

<!-- form markup --!>
```

Taking into consideration how the rest of the library behaves, this behvaiour
was unexpected and meant users would need to explicitly provide default label
classes.

This change makes use of the `HTMLAttributes` and `HTMLClasses` traits so that label
elements build classes as usual —except any custom classes are appended.

For example:

```erb
<%= form_with model: @model do |f| %>
  <%= f.govuk_text_area :details, label: { text: "Some details", class: "special-class" } %>
<% end %>
```

would now output:

```html
<!-- form markup --!>

<label for="model-details-field" class="govuk-label special-class">
  Some details
</label>

<!-- form markup --!?
```

This change in behaviour will affect users of the gem who are customising label
classes but _don't_ want default classes to be applied.

I'm not sure if that is something we need to worry about though, since the form
builder is intended to build GOV.UK (or other brand) forms.

Continuing from the prevoius commit, this change makes the additional
classes passed in via the hint, legend and caption hashes add to the
default rather than overwrite it. This makes it more consistent with the
behaviour in the rest of the library.