Allow localhost api address for backend

[ohmoses]

Closes #726

Allows to specify localHost and localPort in the Publish config—if present, the Publish backend will use this as the API address, while the frontend still uses host and port. E.g.:

"api": {
    "host": "https://api.example.com",
    "port": 443,
    "localHost": "http://localhost",
    "localPort": 8081
}

[eduardoboucas]

Nice and simple! 👌

Can I just request a couple of changes in variable naming:

• apiAddr -> apiAddress: we tend to avoid abbreviations when possible, as it may introduce ambiguity

• localHost -> serverAccessHost, localPort -> serverAccessPort: I understand the reasoning behind using local here, as this feature will most likely be used for accessing an API that is local to the installation of Publish, but this may not necessarily be the case every time; a more encompassing terminology is "server access", as it describes the host/port used when the server accesses the API.

[eduardoboucas]

Why create two separate variables with the result of the Promise and not just:

const {results: languages} = await request(
  `${apiAddress}/api/languages`,
  requestOptions
)