Grouped Backports to the 5.9 branch.

- Editor: Fix Path Traversal issue on Windows in Template-Part Block.
- Editor: Sanitize Template Part HTML tag on save.

Merges [58470], [58471], [58472] and [58473] to the 5.9 branch.
Props xknown, peterwilsoncc, jorbin, bernhard-reiter, azaozz.




git-svn-id: https://develop.svn.wordpress.org/branches/5.9@58483 602fd350-edb4-49c9-b593-d223f7449a82

Author: audrasjb