[Release] 2.1.0

[felipeelia]

• 1. Branch: Starting from develop, cut a release branch named release/2.1.0 for your changes.
• 2. Version bump: Bump the version number in elasticpresslabs.php, /languages/ElasticPressLabs.pot, package-lock.json, package.json, readme.txt, and any other relevant files if it does not already reflect the version being released. In elasticpresslabs.php update both the plugin "Version:" property and the plugin ELASTICPRESS_LABS_VERSION constant.
• 3. Changelog: Add/update the changelog in CHANGELOG.md and readme.txt, ensuring to link the [2.1.0] release reference in the footer of CHANGELOG.md (e.g., 2.0.0...2.1.0).
• 4. Props: Update CREDITS.md file with any new contributors, confirm maintainers are accurate.
• 5. 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.
• 6. New files: Check to be sure any new files/paths that are unnecessary in the production version are included in .gitattributes.
• 7. POT file: Run wp i18n make-pot . lang/elasticpress.pot and commit the file. In case of errors, try to disable Xdebug (see #3079.)
• 8. Release date: Double check the release date in both changelog files.
• 9. Merge: Merge the release branch/PR into develop, then make a non-fast-forward merge from develop into trunk (git checkout trunk && git merge --no-ff develop). trunk contains the stable development version.
• 10. Test: While still on the trunk branch, test for functionality locally.
• 11. Push: Push your trunk branch to GitHub (e.g. git push origin trunk).
• 12. Release: Create a new release, naming the tag and the release with the new version number, and targeting the trunk branch. Paste the release changelog from CHANGELOG.md into the body of the release and include a link to the closed issues on the milestone.
• 13. Close milestone: Edit the milestone with release date (in the Due date (optional) field) and link to GitHub release (in the Description field), then close the milestone.
• 14. Punt incomplete items: If any open issues or PRs which were milestoned for 2.1.0 do not make it into the release, update their milestone to 2.1.1, 2.2.0, 3.0.0 or Future Release.