Make API endpoint naming more consistent

There is some variation in the syntax of the Admin Panel internal REST API.
Endpoints mix singular and plural nouns for collections, as well as hyphens and underscores for multi-word endpoints.
https://restfulapi.net/resource-naming/ provides some good guidance for best practice.

Below are the endpoints that need to be updated (at the time of writing).

Current | New
--- | ---
`/api/planter` | `/api/planters`
`/api/tree_tags` | `/api/tree-tags`
`/api/planter-registration` | `/api/planter-registrations`
`/api/organization` | `/api/organizations`
`/api/organization/{id}/planter` | `/api/organizations/{id}/planters`
`/auth/admin_users` | `/auth/admin-users`
`/auth/check_session` | `/auth/check-session`