Fix on isExternal function (issue #1477)

[GiuliCler]

Summary
This pull request should fix issue #1477 about security bug, still present in the last release.

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[Koooooo-7]

LGTM.

inner fix.

[jhildenbiddle]

Hi @GiuliCler --

I'm closing this PR in favor of #1510. The reason is that this PR doesn't actually "fix" the isExternal function. Instead, the PR modifies isExternal to assume that an incorrectly parsed URL must be external (which is not necessarily the case). #1510 also includes e2e tests.

Thank you for taking the time to file the PR though. Very much appreciated!