Update CHANGELOG.md for 10.4.0

[ci skip]

changelogs/unreleased/41491-fix-nil-blob-name-error.yml

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-title: Fix 500 error when visiting a commit where the blobs do not exist
-merge_request:
-author:
-type: fixed

changelogs/unreleased/41727-target-branch-name.yml

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-title: Set target_branch to the ref branch when creating MR from issue
-merge_request:
-author:
-type: fixed

changelogs/unreleased/41754-update-scss-lint-to-0-56-0.yml

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-title: Update scss-lint to 0.56.0
-merge_request: 16278
-author: Takuya Noguchi
-type: other

changelogs/unreleased/41789-fix-up-web-ide-user-preference-copy-and-buttons.yml

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-title: Fix web ide user preferences copy and buttons
-merge_request: 41789
-author:
-type: other

changelogs/unreleased/41874-closed-todo.yml

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-title: Fix closed text for issues on Todos page
-merge_request:
-author:
-type: fixed

changelogs/unreleased/41882-respect-only-path-in-relative-link-filter.yml

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-title: Ensure that emails contain absolute, rather than relative, links to user uploads
-merge_request: 16364
-author:
-type: fixed

changelogs/unreleased/42025-fix-issue-api.yml

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-title: "[API] Fix creating issue when assignee_id is empty"
-merge_request:
-author:
-type: fixed

changelogs/unreleased/42031-fix-links-to-uploads-in-wikis.yml

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-title: Fix links to uploaded files on wiki pages
-merge_request: 16499
-author:
-type: fixed

changelogs/unreleased/42046-fork-icon.yml

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-title: Fix giant fork icons on forks page
-merge_request: 16474
-author:
-type: fixed

changelogs/unreleased/ac-autodevopfix-kubectl-version.yml

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-title: Force Auto DevOps kubectl version to 1.8.6
-merge_request: 16218
-author:
-type: fixed

changelogs/unreleased/add-tcp-check-rake-task.yml

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-title: Add a gitlab:tcp_check rake task
-merge_request: 15759
-author:
-type: added

changelogs/unreleased/anchor-issue-references.yml

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-title: Fix false positive issue references in merge requests caused by header anchor
-  links.
-merge_request:
-author:
-type: fixed

changelogs/unreleased/api-domains-expose-project_id.yml

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-title: Expose project_id on /api/v4/pages/domains
-merge_request: 16200
-author: Luc Didry
-type: changed

changelogs/unreleased/bump_mysql_gem.yml

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-title: Bump mysql2 gem version from 0.4.5 to 0.4.10
-merge_request:
-author: asaparov
-type: other

changelogs/unreleased/bvl-fork-public-project-to-private-namespace.yml

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-title: Allow forking a public project to a private group
-merge_request: 16050
-author:
-type: changed

changelogs/unreleased/change-issues-closed-at-background-migration.yml

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-title: Use a background migration for issues.closed_at
-merge_request:
-author:
-type: other

changelogs/unreleased/changes-dropdown-ellipsis.yml

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-title: Fixed chanages dropdown ellipsis positioning
-merge_request:
-author:
-type: fixed

changelogs/unreleased/conditionally-eager-load-event-target-authors.yml

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-title: Eager load event target authors whenever possible
-merge_request:
-author:
-type: performance

changelogs/unreleased/da-handle-hashed-storage-repos-using-repo-import-task.yml

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-title: Handle GitLab hashed storage repositories using the repo import task
-merge_request:
-author:
-type: added

changelogs/unreleased/delay-background-migrations.yml

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-title: Run background migrations with a minimum interval
-merge_request:
-author:
-type: changed