fix: oauth user case sensitivity

[matthewelwell]

Changes

Resolves https://github.com/Flagsmith/flagsmith/issues/4185.

The main goal of this PR is to ensure that users who authenticate with an OAuth provider after authenticating with email/password, another OAuth provider, or even SAML (although this use case is slim) get a consistent experience. Currently if the email differs by case, a new user will be created and the user will likely be confused as to why they can't see their projects, etc.

As a side effect, we had to handle a few edge cases where a user might already have duplicate accounts and make sure that the user gets the same experience they do today (authenticating with method A gets them into user A, authenticating with user B gets them user B).

We should, separate to this, get in touch with the users for whom this is an issue (~20) and try to merge their accounts.

How did you test this code?

Added 3 new tests to cover all scenarios.

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 96.52%. Comparing base (e7fd8bd) to head (b4be67a). Report is 9 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #4207 +/- ## ========================================== + Coverage 96.51% 96.52% +0.01% ========================================== Files 1177 1177 Lines 38230 38354 +124 ========================================== + Hits 36896 37023 +127 + Misses 1334 1331 -3 ```

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