Exclude source assets from final package

What does this MR do?

app/assets, ee/app/assets, and vendor/assets are source directories used by the rails asset pipeline, the compiled outputs in public/assets are all that we need to ship to an end-user

Should save ~23Mb based on gitlab-ee@7732acd

$ du -shc app/assets ee/app/assets vendor/assets
 17M    app/assets
2.5M    ee/app/assets
3.3M    vendor/assets
 23M    total

http://guides.rubyonrails.org/asset_pipeline.html https://gitlab.com/gitlab-org/gitlab-ee/blob/7732acd111e63dfb46075e684ae33e025b2f6f0c/config/application.rb#L116

Checklist

See Definition of done.

• Change added to CHANGELOG.md. Not applicable for Documentation changes and minor changes.
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA, if applicable
• MR targeting master branch
• MR has a green pipeline on GitLab.com

Reviewer Checklist

In addition to above, reviewer must:

• Pipeline is green on dev.gitlab.org if the change is not touching documentation or internal cookbooks

References

Closes #3238 (closed)