Add changelog for version 1.3.12

[gudmdharalds]

TODO:

• [x] Define version number in CHANGELOG.md, add release date and update links
• [x] Add same version number in defines.php
• [x] Assign a milestone corresponding to the version number to this PR and PRs that will form the release
• [x] Assign label (Changelog and version)
• [x] Unit-test suite run successful
• [x] Integration-test suite successful (without secrets)
• [x] Run integration-test suite with secrets
• [x] E2E-test suite run successful
• [x] Manual testing
  • [x] Pull request with PHP linting issues
  • [x] Pull request with PHPCS issues
  • [x] Pull request without PHPCS issues, not auto-approved
  • [x] Pull request without PHPCS issues, is auto-approved
  • [x] Pull request with SVG issues
  • [x] Pull request with large file to be skipped
  • [x] Pull request with plugins and themes to be scanned via WPScan API