* DEV: Document external topic id endpoints
This commit documents the existing Create Topic endpoint with the
`external_id` param and documents the new get topic by external id
endpoint.
It also refactors the existing topic show endpoint to use the new format
where we load the expected json schema response from a file.
See: 71f7f7ed49
* clean up unused test variables
When creating a PM target_usernames has been deprecated for some time
now but the api docs have yet to reflect this.
The api docs now specify to use target_recipients.
See: eef21625c6