Add assets.precompile config to Engine

[Spone]

This is a PR to apply the fix suggested by #1816 (comment)

Close #1816

[pablobm]

This looks good to me. I tested it in the following situations:

• Several versions of Rails pre 6.0.
• Rails 6.0.
• Rails 6.0 without Sprockets.
• Rails 6.0 without Webpacker.

Rails pre-6.0 is not affected as it doesn't have Webpacker. For the other three, this PR removes the issue of having to manually update manifest.js when Administrate is installed, and it appears to work well.

My tests were not very comprehensive, but between this and the test suite passing (which includes some JS-dependant code), I feel confident enough that this is a good change.

@nickcharlton Any thoughts? Double checking before I merge as this might be an important change.

[nickcharlton]

This was a bit fun to test! I tried with a script I have to make demo apps but couldn't replicate the original problem but tried it on my current client project where I could. My feeling is that I'm happy with that enough, because clearly this change doesn't break the old implementation and does work where it should.

Thanks for this! I'm going to merge this now.