Updates to mojMenu (Fix #62)

[simonwhatley]

Fix #62

• Rename mojMenu to mojButtonGroup
• Add support for all attributes of govukButton
• Add support for govuk-button--warning type
• Add documentation to component readme
• Add greater number of examples to the frontend app

[adamsilver]

I know we've talked offline but in the spirit of being open I'll summarise my thoughts here:

1. https://inclusive-components.design/menus-menu-buttons/ is worth referencing and covers all sorts of menus and the difference between ‘true menus’

2. I don't think it's right to name the JS component button-group.js because the JS being activated turns it very specifically into a toggle menu—as in it's no longer just a button group.

3. I think the JS behaviour should be changed in accordance with (1). Specifically that it's not a true menu, it's a bunch of links (that happen to look like buttons)—meaning that focus shouldn't move to the first item in the menu when opened. This happens to drastically simplify the script.

4. With (3) said, it could be more ‘true menu’ like if it was used in a multi-select form with multiple actions. The items would be submit buttons (and not links/navigation). But the main use case is (3) and I don't have any evidence that the list being made of submit buttons requires a change of behaviour anyway.

5. All of that to say that I think my favourite name for this is navigation-menu.js, at least for the JS part.