dev/admin: prod build validation

[mastercactapus]

Description:
This PR adds validation of the UI assets when running without the --ui-dir flag. A number of bugs/issues with development targets were also fixed.

• All files under web/src (other than build/ and cypress/) will now trigger app.js to be rebuilt
• Fixed dependency issues where yarn commands would never result in a stable state (always rebuild)
• removed esbuild watcher from Procfiles that already use BUNDLE=1 (they would clobber each other)
• app.js must now exist and the embedded GOALERT_VERSION MUST match the backend version (if --ui-dir is unset)
• esbuild will now abort immediately if GOALERT_VERSION is undefined
• Changing the GIT_VERSION (e.g., by making a commit) will now trigger a rebuild to keep version strings in sync

Additionally, make start-prod is much more efficient now and doesn't print endless noise in the console.