fix(lib): block XSS attacks via nonstandard URLs (#904)

* fix(lib): block XSS attacks via nonstandard URLs

This could allow an attacker to craft an Anubis pass-challenge URL that
forces a redirect to nonstandard URLs, such as the `javascript:` scheme
which executes arbitrary JavaScript code in a browser context when the
user clicks the "Try again" button.

Release-status: cut
Signed-off-by: Xe Iaso <me@xeiaso.net>

* chore: spelling

Signed-off-by: Xe Iaso <me@xeiaso.net>

---------

Signed-off-by: Xe Iaso <me@xeiaso.net>

.github/actions/spelling/allow.txt

@@ -3,4 +3,5 @@ https
 ssh
 ubuntu
 workarounds
-rjack
+rjack
+msgbox

.github/actions/spelling/patterns.txt

@@ -132,3 +132,7 @@ go install(?:\s+[a-z]+\.[-@\w/.]+)+
 # hit-count: 1 file-count: 1
 # microsoft
 \b(?:https?://|)(?:(?:(?:blogs|download\.visualstudio|docs|msdn2?|research)\.|)microsoft|blogs\.msdn)\.co(?:m|\.\w\w)/[-_a-zA-Z0-9()=./%]*
+
+# hit-count: 1 file-count: 1
+# data url
+\bdata:[-a-zA-Z=;:/0-9+]*,\S*