[Peterborough] Allow editing of name field and username field

[MorayMySoc]

Discussion that we should maintain the status quo except for Peterborough

https://mysociety.slack.com/archives/C01TK8P1K8T/p1713950574734509?thread_ts=1713948742.228499&cid=C01TK8P1K8T

https://mysocietysupport.freshdesk.com/a/tickets/4086

[skip changelog]

[codecov[bot]]

Codecov Report

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

Project coverage is 82.90%. Comparing base (d182461) to head (9f0563b). Report is 5 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #4940 +/- ## ========================================== + Coverage 82.60% 82.90% +0.30% ========================================== Files 390 391 +1 Lines 30473 31073 +600 Branches 4812 4999 +187 ========================================== + Hits 25171 25760 +589 - Misses 3864 3875 +11 Partials 1438 1438 ```

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