html/template: escape script tags in JS errors case insensitively

Thanks to Juho Forsén of Mattermost for reporting this issue.

Fixes #70740

Change-Id: I1a49b199dee91cd2bb4df5b174aaa958dc040c18
Reviewed-on: https://go-review.googlesource.com/c/go/+/634696


LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>