Mark rows as even or odd for styling purposes

[kpfefferle]

• Addepar's new visual look includes zebra-striping tables
• Since ember-table uses vertical-collection to occlude rows during scroll, this can't be reliably done with CSS selectors like :nth-child(even) or :nth-child(odd)
• A number of Addepar teams have manually added class="{{if (eq (modulus-of body.rowMeta.index 2) 0) 'is-even' 'is-odd'}}" to their ember-table row component calls to add stable row class names that indicate which rows have an even index vs an odd index
• This PR brings this capability into ember-table itself so that we (and other consumers) don't have to repeat this pattern over and over in order to give ember-tables a zebra stripe styling

[kpfefferle]

This same test failure seems to also be happening on master, so I don't believe it has anything to do with this PR 🤔

[DanMonroe]

Nice, thanks Kevin. This will clean up some code for us.