Add setting for RFS api key #2
Description
We need a new setting for the Rent Fetch Sync API key.
We should put it with the other settings directly related to syncing. We don't need to be able to check this key against the Rent Fetch API yet, but we'll want to have a button separate from the main "save settings" button to check the key asynchronously.
We could go ahead and set up behaviors for the various things that could happen when the key is checked. It could succeed or might not be found, or could already be in use on another site.
On pageload, it should also hit the RF API and validate.