feat: Add logic to API usage notification templates

[zachaysan]

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• I have added information to docs/ if required so people know about the feature!
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Changes

Updated the templates to include logic that is customized towards whether an organisation is a paid account or not.

How did you test this code?

Updated existing tests which had coverage, verifying the free / paid distinction.

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3 Ignored Deployments

Name	Status	Preview	Comments	Updated (UTC)
docs	⬜️ Ignored (Inspect)			Jun 20, 2024 8:24pm
flagsmith-frontend-preview	⬜️ Ignored (Inspect)			Jun 20, 2024 8:24pm
flagsmith-frontend-staging	⬜️ Ignored (Inspect)			Jun 20, 2024 8:24pm

[github-actions]

Uffizzi Preview deployment-53306 was deleted.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.51%. Comparing base (e1f3a7b) to head (b295d39).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4206   +/-   ##
=======================================
  Coverage   96.51%   96.51%           
=======================================
  Files        1177     1177           
  Lines       38290    38293    +3     
=======================================
+ Hits        36956    36959    +3     
  Misses       1334     1334           

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