PULL_REQUEST_TEMPLATE

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## Summary of Changes

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This pull request fixes #<number>

## Submitter Checklist:

- [ ] *Unit Tests* are updated to cover new or changed functionality
- [ ] User-facing strings use `NSLocalizableString()`

## Test Plan:
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## Screenshots:
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## Reviewer Checklist:

- [ ] Issues include necessary QA labels:
  - `QA/(Yes|No)`
  - `bug` / `enhancement`
- [ ] Necessary [security reviews](https://github.com/brave/security/issues/new/choose) have taken place.
- [ ] Adequate unit test coverage exists to prevent regressions.
- [ ] Adequate test plan exists for QA to validate (if applicable).
- [ ] Issue and pull request is assigned to a milestone (should happen at merge time).