Add a second JS pack which can be loaded with the defer attribute about rails-template HOT 7 CLOSED

@eoinkelly I think probably the application pack should be the deferred one. Perhaps we could have another pack named something like "boot" that is non-deferred. This would encourage code to be placed in deferred scripts unless it had to be in the startup pack.

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That's a good idea @joshmcarthur

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Some evidence for defer by default being good: I have recently changed a few of our apps to defer application JS and it mostly just works. The only change I had to make was in an older app but it was an easy refactor.

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Re @joshmcarthur suggestion about a second pack. I was initially in favour of that but I'm not so sure now because I think it would end up being an empty file and a useless HTTP request in production on most apps.

I now think the right approach is:

1. load the main pack with defer
2. Put a big comment at the top of the main pack explaining what that means and how to create a "boot" (non deferred) pack if you need it for some reason

Thoughts?

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Reading https://docs.sentry.io/platforms/javascript/

It’s possible to include defer in your script tag, but keep in mind that any errors which occur in scripts that execute before the browser SDK script executes won’t be caught (because the SDK won’t be initialized yet). We strongly recommend that if you use defer, you a) place the script tag for the browser SDK first, and b) mark it, and all of your other scripts, defer (but not async), thereby guaranteeing that it’s executed before any of the others.

I think we should be OK provided we init Sentry before we load any other JS.

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resolved by #141

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