Update webpack style loaders

[saracope]

Summary

This PR fixes/implements the following bugs/features

• Add postcss loader for autoprefixer (browser prefixing) and css-mqpacker (media query packing)
• Comments out FaviconsWebpackPlugin as it was causing errors. Need to find a replacement for this, see Remove favicons-webpack-plugin #178.
• Fix sourcemaps to show in dev, uses eval-source-map
• Add browserslist config in package.json for postcss processing
• Increased pa11y timeout to 30000ms to fix circleci timeout issue

Explain the motivation for making this change. What existing problem does the pull request solve?

This is in prep for full move to USWDS 2.x which needs vendor prefixing and media query packing.

[saracope]

Pa11y is running locally:

* sc_update-webpack
FCOH2J-K19GHV2F:code-gov-front-end saradcope$ npm run test-pa11y

> @code.gov/code-gov-front-end@1.0.1 test-pa11y /Users/saradcope/Documents/Sites/code-gov-repos/code-gov-front-end
> pa11y-ci --config .pa11yci

Running Pa11y on 14 URLs:
 > http://localhost:8080/projects/nsa_1_atomicwatch - 0 errors
 > http://localhost:8080/projects/gsa_18f_1_10x_product_market_research - 0 errors
 > http://localhost:8080/projects/dod_nci_agency_1_anet - 0 errors
 > http://localhost:8080/browse-projects - 0 errors
 > http://localhost:8080/about/overview/tracking-progress - 0 errors
 > http://localhost:8080/about/overview/introduction - 0 errors
 > http://localhost:8080/open-tasks - 0 errors
 > http://localhost:8080/about/compliance/dashboard - 0 errors
 > http://localhost:8080/about/compliance/procurement - 0 errors
 > http://localhost:8080/about/compliance/inventory-code - 0 errors
 > http://localhost:8080/about/open-source/introduction - 0 errors
 > http://localhost:8080/about/open-source/resources - 0 errors
 > http://localhost:8080/about/open-source/measuring-code - 0 errors
 > http://localhost:8080/about/open-source/licensing - 0 errors

✔ 14/14 URLs passed```

[saracope]

Updated the pa11y timeout to resolve circleci issue.