Add `User.get_by_email` to ensure consistency where we look up a user by their email

[sentry-io[bot]]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:

📄 File: temba/orgs/views.py

Function	Unhandled Issue
pre_process	AttributeError: 'NoneType' object has no attribute 'id' /channels/channel/read/... Event Count: 1

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[codecov[bot]]

Codecov Report

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

Project coverage is 100.00%. Comparing base (146e81e) to head (039e1ce).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #5414 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 503 503 Lines 25162 25166 +4 ========================================= + Hits 25162 25166 +4 ```

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