Address bug with the "Expire custom URLs" not working with "\r\n" line endings

Problem/Motivation

The "Expire custom URLs" command doesn't work on Windows browsers with the "\r\n" line endings applied.
The code was using array_filter rather than array_map.

Proposed resolution

Use array_map rather than array_filter for trimming each line.

Remaining tasks

Submit a patch for it.

User interface changes

• Update the help text to explain exactly how the Expire custom URLs textarea works
• Show a list of expired urls after form submission so that the user can review their action.

API changes

Added a new health check URL endpoint accessible using the new access expire health check permission.
This is useful when working with reverse proxies like Varnish. The path is available at /expire_admin_health_check.