Add lock file for creating packs

[justin808]

ReactOnRails::PacksGenerator.instance.generate_packs_if_stale

Have this call create a lock file so it can't run concurrently. The lock file should expire within a short time.

Here is how to do this:

https://www.perplexity.ai/search/I-maintain-a-QBNsFgY9SKm8qOXU2.N32A