media library acquisition should be at install time, not build time

[cwillisf]

Expected Behavior

All dependencies should be downloaded during npm install or npm ci.

Actual Behavior

Most dependencies are downloaded during npm install or npm ci, except for the media libraries. The media libraries are downloaded during npm run fetch which is part of the dist and distDev build commands.

Steps to Reproduce

1. Clone this repository
2. Run npm install or npm ci
3. Check the static/assets/ directory
4. Run npm run fetch or npm run dist
5. Check the static/assets/ directory again

[cwillisf]

Fixing this could be a step toward fixing #153

[apple502j]

Should be as easy as specifying that as postinstall.