Grouped backports to the 5.6 branch.
- Editor: Bump @wordpress packages for the branch,
- Media: Refactor search by filename within the admin,
- REST API: Lockdown post parameter of the terms endpoint,
- Customize: Escape blogname option in underscores templates,
- Query: Validate relation in `WP_Date_Query`,
- Posts, Post types: Apply KSES to post-by-email content,
- General: Validate host on "Are you sure?" screen,
- Posts, Post types: Remove emails from post-by-email logs,
- Pings/trackbacks: Apply KSES to all trackbacks,
- Mail: Reset PHPMailer properties between use,
- Comments: Apply kses when editing comments,
- Widgets: Escape RSS error messages for display.
Merges [54521-54530] to the 5.6 branch.
Props audrasjb, costdev, cu121, dd32, davidbaumwald, ehtis, johnbillion, johnjamesjacoby, martinkrcho, matveb, oztaser, paulkevan, peterwilsoncc, ravipatel, SergeyBiryukov, talldanwp, timothyblynjacobs, tykoted, voldemortensen, vortfu, xknown.
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