API overhaul: add tests for the API

[josecelano]

Parent issue: #141

• Keep the current API behaviour: URLs and response codes.
• Write tests for all endpoints and all cases for each endpoint.
• Refactor to use one struct for each error (rejection). See this PR.

[josecelano]

hi @da2ce7 @WarmBeer,

We use a token query parameter in the current API version to authenticate API users.
In our case (machine-to-machine communication), I think it's not a good option. Do you want me to use a header in the new version?

From:

curl -X POST http://127.0.0.1:1212/api/key/120?token=MyAccessToken

To:

curl -H 'Authorization: token my_access_token' -X POST http://127.0.0.1:1212/api/key/120

Links

• https://w3ctag.github.io/capability-urls/
• https://developer.github.com/changes/2020-02-10-deprecating-auth-through-query-param/
• https://security.stackexchange.com/questions/118975/is-it-safe-to-include-an-api-key-in-a-requests-url
• https://stackoverflow.com/questions/643355/https-url-with-token-parameter-how-secure-is-it