Improve bundling of ui dependencies via gulp build

[danjm]

In #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 <script src="./libs.js" type="text/javascript" charset="utf-8"></script> line in home.html, popup.html and notification.html is only included in production build.

[Gudahtt]

The first improvement would probably be best to leave until later, as there are planned changes to our build system that would make that obsolete (this hasn't been made into an issue yet - that's in progress).

The second point is worth addressing now though, if only to get rid of that error about libs.js being missing in development. The approach described in the OP would certainly work, but one alternative might be using the lib.js bundle in development like we do in production.