DO NOT MERGE Fix #149 Heroku doesn't pick up changed environment variables

[osteele]

Move the webpack build step from postinstall to start.

This makes bringing up a dyno slower (although the build is faster, so it should be the same total time for a new deployment), but the app picks up changes to environment variables without requiring a re-deploy.

This also brings another couple of benefits:

• yarn start runs the latest sources. This is typically what I want; the fact that it doesn't has got me a few times. (yarn run is available to just run the production server, without building the client.)
• yarn install no longer runs webpack. This was undesireable behavior (although fixing just this could have been accomplished by moving the build from postinstall to heroku-postbuild).

Description

Describe your changes here.

Required

Changes must conform to these requirements:

• yarn test passes. All new and existing tests pass.
• yarn lint passes. All new code follows the code style of this project.

Aspirational

We don't yet require these, but they are nice to have:

• New code is covered by new or existing tests.
• Changed code is covered by new or existing tests.

[coveralls]

Pull Request Test Coverage Report for Build 341

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 33.609%

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Totals
Change from base Build 337:	0.0%
Covered Lines:	157
Relevant Lines:	461

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💛 - Coveralls