Grouped backports to the 5.1 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,
- Widgets: Escape RSS error messages for display.
Merges [54521-54530] to the 5.1 branch.
Props voldemortensen, johnbillion, paulkevan, peterwilsoncc, xknown, dd32, audrasjb, martinkrcho, vortfu, davidbaumwald, tykoted, timothyblynjacobs, johnjamesjacoby, ehtis, matveb, talldanwp.
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