Fix Broken Socket on Client for Custom/Random Port Numbers

[shellscape]

What kind of change does this PR introduce?
Bugfix

Did you add or update the examples/?
No need

Summary
#1059 exposed a situation in which the sockjs url for clients with custom/random ports would result in the client using an erroneous url that either omitted the port or used the wrong (default 8080) port. This resulted in socket communication breaking, which was noticed by way of the lack of an overlay being displayed.

This fix does the following:

1. in Server.js the socket server won't be initialized nor started until the listeningApp triggers the callback on listen. That's really the proper time for the socket server to be initialized anyhow. polyfills dep is also moved to the top to match bin/webpack-dev-server.js.
2. in client/index.js the port parsing is going to look for null and '0' on urlParts.port and that'll be refactored slightly.
3. in bin/webpack-dev-server.js the polyfills dep is added to the top of the file preventing internal-ip errors on Node 4.x.

Does this PR introduce a breaking change?
Negatory

Other information

[codecov]

Codecov Report

Merging #1060 into master will decrease coverage by 0.04%.
The diff coverage is 37.03%.

@@            Coverage Diff             @@
##           master    #1060      +/-   ##
==========================================
- Coverage   73.37%   73.33%   -0.05%     
==========================================
  Files           5        5              
  Lines         447      450       +3     
  Branches      141      142       +1     
==========================================
+ Hits          328      330       +2     
- Misses        119      120       +1

Impacted Files	Coverage Δ
lib/Server.js	80.12% <37.03%> (-0.13%)	⬇️

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