Distributor: Release version 1.6.0

This issue is for tracking changes for the 1.6.0 release. Target release date: TBD.

Pre-release steps

• [ ] review and (merge or punt) open PRs in the milestone

Release steps

• [x] Branch: Starting from develop, cut a release branch named release/1.6.0 for your changes.
• [x] Version bump: Bump the version number in distributor.php, distributor.pot, and readme.txt if it does not already reflect the version being released. In distributor.php update both the plugin "Version:" property and the plugin DT_VERSION constant, ensuring that it is suffixed with -dev.
• [x] Changelog: Add/update the changelog in CHANGELOG.md.
• [x] Props: update CREDITS.md file with any new contributors, confirm maintainers are accurate.
• [x] Readme updates: Make any other readme changes as necessary. README.md is geared toward GitHub and readme.txt contains WordPress.org-specific content. The two are slightly different.
• [x] Translations: Update the .pot file by running npm run makepot.
• [x] Merge: Make a non-fast-forward merge from your release branch to develop (or merge the pull request), then do the same for develop into master (git checkout master && git merge --no-ff develop). master contains the stable development version.
• [x] Build: In the master branch, run npm install && npm run release. This will create a subfolder called release with the stable branch cloned into it as a worktree and latest changes copied over. Ensure that any new files are in the release folder; if not, you may need to add them to gulp-tasks/copy.js.
• [x] Check: Are there any modified files in master? If so, head back to develop, run all necessary tasks and commit those changes before heading back to Step 6.
• [x] Test: Switch to running Distributor from the version in the release subfolder and run through a few common tasks in the UI to ensure functionality.
• [x] Push: First master: git push, then from within the release directory, add all files and push them to origin stable: git push origin stable.
• [x] Release: Create a new release, naming the tag and the release with the new version number, and targeting the stable branch. Paste the changelog from CHANGELOG.md into the body of the release and include a link to the closed issues on the 1.6.0 milestone. The release should now appear under releases.
• [ ] Version bump (again): In the develop branch (cd ../ && git checkout develop) bump the version number in distributor.php, distributor.pot, and readme.txt to 1.6.1-dev. It's okay if the next release might be a different version number; that change can be handled right before release in the first step, as might also be the case with @since annotations.
• [x] Close milestone: Edit the 1.6.0 milestone with release date (in the Due date (optional) field) and link to GitHub release (in the Description field), then close the milestone.
• [x] Punt incomplete items: If any open issues or PRs which were milestoned for 1.6.0 do not make it into the release, update their milestone to 2.0.0 or Future Release.

Post-release steps

• [ ] submit Distributor to Packagist (see #356)