Reduce webpack dev server manifest calls

What does this MR do?

Reduce webpack dev server manifest calls

• Reduces the number of GETs for the manifest.json from the webpack dev server from 6 to 1.

While troubleshooting an issue where my GDK was taking 30+ seconds before first byte on any request through rails-web, @ali-gitlab and I found that we were doing an HTTParty.get for the manifest.json from the webpack dev server 6 times, because no caching is employed when using the webpack dev server. We changed this to use the Gitlab::SafeRequestStore to cache the manifest on a per-request basis.

This reduced time to first byte from 30+ seconds to 5+ seconds.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team