Make API endpoint naming more consistent

[nmcharlton]

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

[nmcharlton]

We're in the process of moving to a new set of API microservices that will eventually replace much of the existing Admin Panel API, so this standardisation is of low priority.