v0.7.3 released

I’ve managed to avoid mostly working on bandcrash for a few days, as I keep promising myself. However, a little bug came up that led me to make another release. Go figure.

Fixes since 0.7.2:

• The user CSS file actually gets applied to the player now
• Fixed the macOS Universal build for reals this time (I hope) and added a test to make the build fail if it ever breaks again
• Removed a dependency on a library that was causing the Universal build to break
• Improved the docs for player hosting
• Fixed broken track drag-and-drop