1.0.1 hardening release

Adds element-level validation that restricts the LSCache Purger
purge_host field to http:// and https:// schemes. Drupal's url
element type otherwise accepts any scheme PHP's filter_var
recognises (file, ftp, ldap, gopher, etc.). The HTTP purger only
makes sense over http or https; other schemes would have failed at
runtime anyway, but the new validator surfaces an immediate error
and closes a (very narrow) avenue for an admin with the
'administer lscache' permission to use the form to probe internal
endpoints.

Operator-facing only; permission-gated. No security advisory
associated as no exploitable vulnerability was identified. Defence
in depth from the 1.0.0 pre-stable security review.