Fix several vulnerabilities (#13927)

Fix several vulnerabilities (#13927)

* Escape HTML in the location field of a calendar event post

- This allowed script tags to be interpreted in the post display of an event.

* Add form security token check to /admin/phpinfo module

- This prevents basic XSS attacks against /admin/phpinfo

* Add form security token check to /babel module

- This prevents basic XSS attacks against /babel

* Prevent pass-through for attachments

- This addresses a straightforward Reflected XSS vulnerability if a malicious HTML/Javascript file is attached to a post through upload

* Prevent overwriting cid on event edit

- This allowed to share an event as any other user after zeroing the cid field of an existing event