Send notification to admins re: FSA approval; fix amount in FSA completion email

[matthew-li]

Description

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• Added logic to send an email to a configured list of administrators when an FSA request is approved (and ready for processing).
• Renamed FSA notification service methods for clarity.
• Updated the email sent to users when an FSA request is completed to include the approved amount, not the requested amount.
• Updated tests.

Type of change

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• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

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• Ensure that the test suite passes on GitHub Actions.

PR Self Evaluation

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• My code follows the agreed upon best practices
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (if needed)
• My changes generate no new warnings
• Any dependent changes have been merged and published in the appropriate modules
• I have performed a self-review of my own code