feat: #462 用户更新议题 assignee

...odules/community/models/community_post_details_model.dart

lib/modules/issues/details/issue_details_page.dart

lib/modules/issues/details/models/issue_details_model.dart

lib/modules/issues/details/widgets.dart

lib/modules/issues/manage/issue_creation_page.dart

...modules/issues/manage/models/assignee_selector_model.dart

lib/modules/issues/manage/models/issue_creation_model.dart

lib/modules/issues/manage/models/issue_update_model.dart

...gration_tests/modules/issues/test_issue_details_page.dart

...ntegration_tests/modules/projects/projects_page_test.dart

...ion_tests/modules/projects/test_subgroup_issues_page.dart

test/integration_tests/view_linked_issue_test.dart

...s/community/models/community_post_details_model_test.dart

test/modules/issues/issue_creation_model_test.dart

test/test_data/issue.dart