chore(refactor): Make changes based on review

[NickColley]

Sorry this is a big diff, hopefully it's ok with you @espy, let me know!

• Adds classes instead of styling core elements
• Styles the signin form so it's responsive etc
• Use 'js-*' prefixed classes
• Use the 'hide' class instead of inline style

[espy]

Thanks, very neat :) I'll add in the hover states for the buttons again, and sort out the table cell widths in a minute.

[NickColley]

@espy :+1: I think with the hover styles if we add them back we need to make them distinct from focus so as to not confuse any users that they are focused on an element when they're not.

[espy]

Good point!