chore: OPTIC-1061: Improve webpack bundle management so bundles can be cached more effectively

[bmartel]

PR fulfills these requirements

• Commit message(s) and PR title follows the format [fix|feat|ci|chore|doc]: TICKET-ID: Short description of change made ex. fix: DEV-XXXX: Removed inconsistent code usage causing intermittent errors
• Tests for the changes have been added/updated (for bug fixes/features)
• Docs have been added/updated (for bug fixes/features)
• Best efforts were made to ensure docs/code are concise and coherent (checked for spelling/grammatical errors, commented out code, debug logs etc.)
• Self-reviewed and ran all changes on a local instance (for bug fixes/features)

Change has impacts in these area(s)

(check all that apply)

• Product design
• Backend (Database)
• Backend (API)
• Frontend

Describe the reason for change

Defining app css block so it can be overridden upstream for redefinition of assets.

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[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 14 lines in your changes missing coverage. Please review.

Project coverage is 76.77%. Comparing base (4dd9e7a) to head (b63bd6c).
Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
label_studio/core/utils/manifest_assets.py	56.25%	7 Missing ⚠️
label_studio/core/utils/static_serve.py	25.00%	6 Missing ⚠️
label_studio/core/templatetags/filters.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #6797      +/-   ##
===========================================
- Coverage    76.83%   76.77%   -0.06%     
===========================================
  Files          170      171       +1     
  Lines        13994    14021      +27     
===========================================
+ Hits         10752    10765      +13     
- Misses        3242     3256      +14     

Flag	Coverage Δ
pytests	76.77% <50.00%> (-0.06%)	⬇️

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