fix(webpack): externalize server source-map to prevent RangeError

[ZauberNerd]

For some reason we see intermittent failures with the source-map-support
package (examples: evanw/node-source-map-support#252
and evanw/node-source-map-support#93).
The fix that worked for us seems to be to externalize the source-map.

Bors merge bot cheat sheet

We are using bors-ng to automate merging of our pull requests. The following table provides a summary of commands that are available to reviewers (members of this repository with push access) and delegates (in case of bors delegate+ or bors delegate=[list]).

Syntax	Description
bors merge	Run the test suite and push to master if it passes. Short for "reviewed: looks good."
bors merge-	Cancel an r+, r=, merge, or merge=
bors try	Run the test suite without pushing to master.
bors try-	Cancel a try
bors delegate+	Allow the pull request author to merge their changes.
bors delegate=[list]	Allow the listed users to r+ this pull request's changes.
bors retry	Run the previous command a second time.

This is a short collection of opinionated commands. For a full list of the commands read the bors reference.