Improve bundling of ui dependencies via gulp build

Nobody has claimed this yet.

In https://github.com/MetaMask/metamask-extension/pull/5547, we started bundling some front end dependencies separately from the rest of our UI code.

The purpose of this was to shrink the size of the largest file in our bundle.

There were two noted improvements in that PR.
- dynamically sourcing the list of dependencies or files to include in separate bundles
- dynamically inserting script tags into the appropriate *.html files

To complete the first, we need to dynamically source all files that should be in the bundle and then bundle them into a number of files of roughly equal size. Package names are currently hard-coded near the top of the gulp file.

To complete the second, we need to ensure that the `` line in `home.html`, `popup.html` and `notification.html` is only included in production build.

Read PR 5547 and the gulp file, where the package names are currently hard-coded. Then inspect home.html, popup.html, and notification.html; done means dependencies are sourced dynamically into roughly equal bundles and the libs.js script is present only in production builds.

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