Only de-duplicate on primary email address

[Andrew-Dickinson]

Removes de-duplication on phone numbers and non-primary email addresses, as this creates some data integrity and safety concerns. We now only dedupe on the email address entered in the join form, which results in a noisier database, but is a safe and simple choice until we are able to better handle members without email addresses

Also re-enables the requirement that join form submissions include an email address

Closes #550

[codecov[bot]]

Codecov Report

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

Project coverage is 94.18%. Comparing base (dc2f0f3) to head (0714708). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #641 +/- ## ========================================== - Coverage 94.22% 94.18% -0.05% ========================================== Files 80 80 Lines 3258 3251 -7 ========================================== - Hits 3070 3062 -8 - Misses 188 189 +1 ```

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