perf: don't unneccessarily rebuild dev assets

[kdmccormick]

Background

Currently, Tutor Dev users who mount edx-platform end up needing to install node_modules and build static assets three separate times!

1. in the openedx (prod) image build, which openedx-dev is based on
2. in the openedx-dev image build
3. in mounted-directories.sh

In addition, devs are forced to collect assets and pull translations as part of step 1, which is slow and not necessary for dev mode.

Even though the artifacts from steps 1-2 are totally ignored when bind-mounting edx-platform, those steps are re-executed every time any part of the image is to be rebuilt, e.g. adding/removing dev python packages. This is a huge time sink for edx-platform devs and package devs.

This PR

This PR makes a few critical changes:

• Split each type of asset compilation (js/css X dev/prod) into its own minimal stage, eliminating dev-prod redundancy and avoiding spurious asset rebuilds.
• Pull translations and collect assets for prod, but not for dev.
• Generate assets (js, css, and node_modules) into the openedx image outside the edx-platform repo so that bind-mounts do not clobber them.
• Use the new ln-assets script to create links from edx-platform to those artifacts.

The combined result of all of these changes is that you should only need to build static assets once, whether you're building prod or dev, whether you're bind-mounting edx-platform or not. You should only need to rebuild assets if you actually make changes to the asset sources (js, css, underscore, images, etc.).

Trying out this branch?

If you're trying this branch, I'm assuming that you are bind-mounting edx-platform and using tutor dev.

This is a based on nightly, make sure you have a recent-ish version of edx-platform master. It must include this change, merged early June.

You could also cherry-pick this Tutor commit onto Tutor master and try it out with edx-platform Redwood. You'll need a very recent version of redwood.master, including this change, merged mid-June.

Then, run:

tutor dev stop
tutor config save
tutor images build openedx-dev
tutor dev do init --limit=lms  # Will create the symlinks in edx-platform
tutor dev start -d

Getting back off this branch?

If you're switching back to upstream Tutor master or nightly, and you were using a bind-mounted edx-platform, then you'll want to remove the symlinks from your edx-platform bind-mount so that they don't interfere. From edx-platform, run:

• make clean

Then:

tutor dev stop
tutor config save
tutor images build openedx-dev
tutor dev do init --limit=lms  # will re-generate your bind mount's artifacts, the old way
tutor dev start -d