[refactor] Move row meta to CollapseTree

[pzuraq]

This PR refactors CollapseTree to take much more of the responsibility
of managing the state of rows, and setting up their meta objects. This
does a few things:

• Normalizes the tree structures - CollapseTree now behaves much like
  ColumnTree. At some point they could probably be made to extend from
  the same base class.
• Gives us enough context to correctly toggle selection (which allowed
  us to fix selection, and add the grouping mode)

API Changes

• ember-tbody no longer accepts a tree object, it'll only take an
  array of rows. To get a single root node in a tree, pass in an array
  with a single parent node.
• collapsed -> isCollapsed on nodes to bring it inline with
  terminology through the repo
• New selectMode option: grouping
• Toggling collapse state is now done by a checkbox is the first cell.
  Both checkboxes and the table itself will be styled to match Addepar's
  internal table usage in an upcoming PR.
• Tree mode and collapsing are now toggleable via enableTree and
  enableCollapse on the tbody.

Other Stuff

• Docs added for the tbody
• Lots of new tests added!

Note: There is a subtle memory leak introduced in this PR, would like to fix it before we merge but this one is really tricky. I've narrowed it down to the onSelect action, somehow by sending rows up from there we're leaking.

[addepar-andy]

Toggling collapse state is now done by a checkbox is the first cell.

why a checkbox? that isn't how the analysis table works

[pzuraq]

We can style a checkbox to look just like the toggle icon that exists in our internal tables today. Using a checkbox is more semantically correct and gives us a lot of behavior and a11y for free.