Markup / tabindex / consistency fixes

[ErisDS]

refs #5652

Fixes:

Pressing "tab" after the last input field takes keyboard focus to address bar. Should be: submit button.

Providing that this option is chosen in safari:

This is WIP because there are more places I'd like to test / check this yet - just getting it open to show progress.

A couple of questions:

• Are the consistency changes wanted? I think using a block helper for gh-spin-button, makes more sense / is easier to read, but not sure what the ember take on this is. Also I think it makes sense not to include 'type="button"' when that is the default.
• Where buttons are submit button near a form (e.g. on setup/two and setup/three) should they not be inside the form? I thought that was normal from a markup perspective, but maybe it makes styling hard?

What's in the box:

• ensure gh-spin-button passes type & tabindex through to markup
• add gh-input class to auto focus input
• add tabindexes to setup/two + make first field autofocus and button submit
• always put button text inside opening/closing helper tag
• only include type if it is 'submit' as button is default
• wrap attributes in double quotes

[JohnONolan]

👍