Merge pull request #300 from ahinz/fix/210/accordions

Make map page accordion work correctly. Fixes #210.
Merge pull request #295 from ahinz/fix/150/show-error-on-duplicate-registration

Show error on duplicate registration
Allow accessing tree via nested urls. Fixes #134.

Now you can do:

`/instance/plot/{plot_id}`
and
`/instance/plot/{plot_id}/trees/{tree_id}`

If `tree_id` isn't a tree on `plot_id`, it will return 404.