Fix 502 errors due to IPv6

[Wielewout]

Description

This does the same as #1149, there were only some merge conflicts in that PR preventing it to be merged.
So taking over that description:

This changes how the NGINX proxy server resolves to the underlying sharelatex service. The change is to use "localhost" instead of the IPv4 address 127.0.0.1. There is an inconsistency in the current configuration as the App.js for sharelatex binds to "localhost" not 127.0.0.1, so if localhost resolves to the IPv6 address ::1, then there's a misconfiguration between nginx and the node.js server.

To fix this, either we make a change in the app.js for sharelatex to always force IPv4 by providing a 127.0.0.1 , or we just use whatever localhost binds to in both. For me, I think it is more logical to bind to localhost consistently, allowing the use of IPv6 or IPv4, whatever is preferred by the underlying system for "localhost".

Related issues / Pull Requests

Fixes #1146
Related to #1149

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[Doomsdayrs]

@aeaton-overleaf Can this be reviewed?