Leverage manifest file to improve startup speed.

[Archmonger]

Description

Currently, the boot time of ServeStatic can be pretty slow if there's thousands of static files due to needing directory traversal and calling os.stat on each file.

This PR aims to minimize startup time by reading Django's manifest file when possible.

Derivative of upstream PR:

• Added Django Middleware that Delays Traversal of Static Root until Requested evansd/whitenoise#275

Checklist

Please update this checklist as you complete each item:

• Tests have been developed for bug fixes or new functionality.
• The changelog has been updated, if necessary.
• Documentation has been updated, if necessary.
• GitHub Issues closed by this PR have been linked.

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