Faster Webpack builds

Created by: sqs

This PR significantly speeds up Webpack initial and incremental builds of TypeScript and SCSS:

• 25% faster initial build time (31s -> 23s)
• 74% faster TypeScript incremental builds (3.7s -> 1.0s)
• 47% faster SCSS incremental builds (5.5s -> 2.9s)

See commit messages for a discussion of each change.

Benchmarks:

• Initial build: goreman run restart web and look for Time: ...
• TypeScript incremental build: echo '/* a */' >> web/src/SourcegraphWebApp.tsx and look for Time: ...
• SCSS incremental build: echo '/* a */' >> web/src/SourcegraphWebApp.scss and look for Time: ...

(All on the same computer, with nothing else running.)

master branch:

• Initial build: 31067ms
• TypeScript incremental build: 3684ms
• SCSS incremental build: 5519ms

faster-webpack branch:

• Initial build: 23310ms (-25%)
• TypeScript incremental build: 966ms (-74%)
• SCSS incremental build: 2938ms (-47%)