fix: Avoid a long table lock for the first_name migration

[khvn26]

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Changes

This replaces users.0039 migration with a manual one that aims to avoid long locking the users_ffadminusers table during migration execution.

A new users_ffadminusers.first_name_v2 column is added and is backing the FFAdminUser.first_name field.

Notes:

• Should we replace the migraton or rather add one? Currently, to deploy this on top of the previous version, a migrations table mofification is in order.
• Alternatively, we could probably do away with adding ALTER TABLE {table_name} RENAME COLUMN first_name_v2 TO first_name statement to the forward migration.
• This is most likely not compatible with Oracle due to syntax differences.

How did you test this code?

Added two unit tests for reverse and forward migrations.

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3 Skipped Deployments

Name	Status	Preview	Comments	Updated (UTC)
docs	⬜️ Ignored (Inspect)			Nov 6, 2024 6:09pm
flagsmith-frontend-preview	⬜️ Ignored (Inspect)			Nov 6, 2024 6:09pm
flagsmith-frontend-staging	⬜️ Ignored (Inspect)			Nov 6, 2024 6:09pm

[github-actions]

Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-api-test:pr-4816	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-e2e:pr-4816	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-frontend:pr-4816	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-api:pr-4816	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-4816	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-4816	Finished ✅	Results ✅

[github-actions]

Uffizzi Ephemeral Environment deployment-58038

☁️ https://app.uffizzi.com/github.com/Flagsmith/flagsmith/pull/4816

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.33%. Comparing base (364c565) to head (78b666a).
Report is 11 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4816   +/-   ##
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  Coverage   97.33%   97.33%           
=======================================
  Files        1180     1181    +1     
  Lines       41213    41251   +38     
=======================================
+ Hits        40113    40151   +38     
  Misses       1100     1100           

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[matthewelwell]

Alternate implementation here: #4817