Lodash require path change

For #5637:
* Rename lodash.custom.min.js to lodash.js so our JS code hinting finds it.
* Remove the special require configuration for it so it's more consistent with other require paths that have to be relative to src. This means you have to do `require("thirdparty/lodash")` instead of just `require("lodash")`. (We might want to do a more general cleanup of this in the future, but for now we wanted to keep it consistent.)