Feature/general accordion #173303343

[LoganArnett]

General Accordion and Accumulator PR for ticket #173303343

Accordion

Accordion with long text and text edit

Accordion header (with selected icon)

Accumulator: with two selections

Accumulator: with no selections

[james-wills-sf]

Fixed some of the accordion styling, this is what it looks like now:

TODO:

• [x] Add example with "selected" accordion header.
• [x] Fix padding between accordion cells + content border
• [x] Add "other" checkbox with text field
• [x] polish accumulator pattern

[james-wills-sf]

@akegan this should be ready to review now!

[james-wills-sf]

Review app: https://sf-dahlia-pl-accordion.herokuapp.com/components/detail/accordion-plus-minus.html

[james-wills-sf]

@akegan if you want to take a look I finished all of the polish pass changes. I'm currently deploying the review app now, when it's ready it should be at https://sf-dahlia-pl-accordion.herokuapp.com/components/detail/accordion-plus-minus.html