Deconstruct client build process

[natefoo]

This adds some transparency for the admin but requires us to synchronize with the client build scripts and will be complicated by differences across versions. Because of that, the default is still to invoke make client-production-maps.

[dannon]

If you did want to separate the (new, and likely growing) plugin building/staging from the other gulp stuff that's central to the actual client build you could break it down like this:

yarn run gulp fonts && yarn run gulp stageLibs
yarn run gulp plugins

Default does all of that, and for simplicity the 'plugins' one is lumped into the same process, but I wasn't sure if you'd want to separate those concerns a little more in this decomposition. Doesn't matter to me.

edit: Just submitted galaxyproject/galaxy#8742 after reviewing the targets and thinking about it a bit in this context. You'll want to take that into account I guess.

[natefoo]

Thanks for the review and suggestions!