Use API proxy when Playgrounds option is on

[nataliebrotherton]

src/class-wa-api.php

• Clean up class constants
• Add proxy URL constant
• Create functions (get_auth_url,get_api_url, and get_public_api_url) that conditionally return the WildApricot API url or the proxy url if the playgrounds constant is enabled

assets/blueprint.json

• Add config constant WP_PLAYGROUNDS_SUPPORT to indicate playgrounds is being used.
• Temporarily change plugins url to point to this branch for testing