escapeButPreserveUris() matched the scheme with "[A-z]+?://", which accepts
any scheme. A frame comment containing javascript://x.co/?%0Aalert(1) survives
htmlspecialchars() untouched and was then wrapped in an anchor, so clicking it
executed script in the origin of the error page. "[A-z]" is also a code point
range rather than a letter class, so it additionally matched the six characters
between "Z" and "a", making "_://x.co/a" a link too.
Match the scheme with "https?" instead, case-insensitively so uppercase
"HTTP://" keeps working. Other schemes stay as escaped plain text. The editor
integration links are not affected, they are emitted directly by the template
and never pass through this method.
Fixes#791
