Fix host and port for front-end

[paulodiovani]

Describe the bug

We need to have a better control over env vars to allow the project to run on a web server. In short, we need a way to inform the correct backend url.

Changes to do

Update src/api.js and set:

• host to process.env.VUE_APP_DEFAULT_BACKEND_HOST || 'http://localhost'
• port to process.env.VUE_APP_DEFAULT_BACKEND_PORT || 8001
• axios.get(${host}:${port}/api/v1/pulls/${year})
• add VUE_APP_DEFAULT_BACKEND_HOST to .env.example

[paulodiovani]

Reopened, I notice an issue when using port 80 on heroku. Probably on other servers too:

• In src/api.js
  • if port is 80 (parseInt or compare as string), do not add port to the endpoint (eg: ${host}/api/v1/pulls/${year}
• In .env.example
  • change to VUE_APP_DEFAULT_BACKEND_HOST=//localhost (not http(s), so we don't need to care about the protocol).