Fix image processing

[saracope]

Summary

This PR implements the following changes:

• remove the set-env task from gulpfile.js (was not working) and references to it in package.json
• sets the node_env to production when running on federalist
• add a check for the development environment before loading image tasks
• adds a watch task to monitor the content/images/_inbox folder for changes; when a change is made (image added) it will kick of the img task to process and upload the image
• add dist folder to the .gitignore file so we're not asked to commit any changes that get compiled there
• update uswds.js and the browsers list that was breaking due to recent changes on uswds-gulp

Documentation has also been updated:
https://github.com/GSA/digitalgov.gov/wiki/Adding-Images

Testing
From your project repository, bring in the changes to test:
first be sure you have the lastest production build

• git pull origin master
• git fetch origin

then checkout this branch

• git checkout -b sc_fix-gulp-img origin/sc_fix-gulp-img
  and merge it with master so you're testing the most up-to-date version
• git merge master

and spin the platform up locally and test with images

• run npm start
• add a test image to the content/images/_inbox folder
• verify that the img script ran and the appropriate files were created

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