Revert "Revert "Proxy webpack dev server in test environment""

What does this MR do and why?

This restores the changes in !79457 (merged) by reverting a2ba70ec.

The root cause should have be solved by the upgrade to rack-proxy (!83241 (merged)). This upgrade should prevent chunked transfers from being proxied through Workhorse.

How to set up and validate locally

I wasn't able to reproduce #352649 (closed) even after I downgraded rack-proxy to v0.6.0.

With this branch, this should pass:

bundle exec rspec --fail-fast spec/features/groups_spec.rb spec/features/projects/new_project_spec.rb spec/features/merge_request/user_sees_pipelines_spec.rb ee/spec/features/issues/user_uses_quick_actions_spec.rb

Relates to #352649 (closed)

MR acceptance checklist

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