Use COMMUTER_PORT variable everywhere

[Greg Roodt]

At the moment there is inconsistency between the variables the frontend uses for knowledge about the server PORT and the server itself.

See:

commuter/pages/view.js

Line 45 in a69c08b

 const port = process.env.COMMUTER_PORT || 4000; 

vs

commuter/backend/config.js

Line 139 in 146a5eb

 config.port = process.env.PORT || process.env.COMMUTER_PORT || 4000; 

I think we should only use COMMUTER_PORT or default to 4000 everywhere.

I'll raise a PR to fix this shortly.