Allow attendees to modify their data (Z#23152886)

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 89.06250% with 7 lines in your changes are missing coverage. Please review.

Project coverage is 78.09%. Comparing base (aa55eb2) to head (3ec640b).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #4138 +/- ## ========================================== + Coverage 78.08% 78.09% +0.01% ========================================== Files 429 429 Lines 60333 60397 +64 ========================================== + Hits 47109 47168 +59 - Misses 13224 13229 +5 ``` | [Files](https://app.codecov.io/gh/pretix/pretix/pull/4138?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pretix) | Coverage Δ | | |---|---|---| | [src/pretix/api/serializers/event.py](https://app.codecov.io/gh/pretix/pretix/pull/4138?src=pr&el=tree&filepath=src%2Fpretix%2Fapi%2Fserializers%2Fevent.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pretix#diff-c3JjL3ByZXRpeC9hcGkvc2VyaWFsaXplcnMvZXZlbnQucHk=) | `93.01% <ø> (ø)` | | | [src/pretix/base/services/placeholders.py](https://app.codecov.io/gh/pretix/pretix/pull/4138?src=pr&el=tree&filepath=src%2Fpretix%2Fbase%2Fservices%2Fplaceholders.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pretix#diff-c3JjL3ByZXRpeC9iYXNlL3NlcnZpY2VzL3BsYWNlaG9sZGVycy5weQ==) | `92.00% <ø> (ø)` | | | [src/pretix/base/settings.py](https://app.codecov.io/gh/pretix/pretix/pull/4138?src=pr&el=tree&filepath=src%2Fpretix%2Fbase%2Fsettings.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pretix#diff-c3JjL3ByZXRpeC9iYXNlL3NldHRpbmdzLnB5) | `95.91% <ø> (ø)` | | | [src/pretix/control/forms/event.py](https://app.codecov.io/gh/pretix/pretix/pull/4138?src=pr&el=tree&filepath=src%2Fpretix%2Fcontrol%2Fforms%2Fevent.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pretix#diff-c3JjL3ByZXRpeC9jb250cm9sL2Zvcm1zL2V2ZW50LnB5) | `87.38% <ø> (ø)` | | | [src/pretix/base/models/orders.py](https://app.codecov.io/gh/pretix/pretix/pull/4138?src=pr&el=tree&filepath=src%2Fpretix%2Fbase%2Fmodels%2Forders.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pretix#diff-c3JjL3ByZXRpeC9iYXNlL21vZGVscy9vcmRlcnMucHk=) | `89.19% <91.30%> (+0.09%)` | :arrow_up: | | [src/pretix/presale/views/order.py](https://app.codecov.io/gh/pretix/pretix/pull/4138?src=pr&el=tree&filepath=src%2Fpretix%2Fpresale%2Fviews%2Forder.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pretix#diff-c3JjL3ByZXRpeC9wcmVzYWxlL3ZpZXdzL29yZGVyLnB5) | `82.80% <87.80%> (+0.30%)` | :arrow_up: |