User-supplied custom realm filter has had some sort of regex-based
validation of the format URL since their introduction in
d7e1e4a2c0 -- and this has always been
in addition to the URLValidator. The URLValidator is the one which
does the security-relevant work of validating that the schema is
reasonable, and that the overall shape of the URL is well-formed. The
regex has served primarily to arbitrary limit the characters that can
appear in the URL, in the mistaken name of safety.
Adjust the regex, such that its only purpose is to verify that the
usages of `%` characters in the URL are reasonable, and leave the URL
validation to the URLValidator, which can do a far better job. This
includes broadening the support to include `%%` as an escape
character; this is likely such a niche case as to be unnecessary, but
costs little.
Fixes#16013.
8dd9b4e812