G2-MMS: Update/Add central mess backend workflows

[Zapper9982]

Issue that this pull request solves

Closes: N/A

Proposed changes

Brief description of what is fixed or changed

This PR adds and updates the Central Mess backend workflows, including:

• Added Central Mess API endpoints and URL routing.
• Added serializers and API views for mess operations.
• Added request workflows for registration, deregistration, payment updates, rebates, and special food requests.
• Added warden decision/escalation flow.
• Added mess announcements and menu poll support.
• Added new database migrations for special request rules, warden decision flow, and announcements.
• Updated notification support for mess-related workflows.
• Added backend tests for Central Mess APIs and workflows.

Types of changes

Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Other (please describe):

Checklist

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• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have created new branch for this pull request
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation

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