Add test to validate all l10n source messages with HTML Purifier

[andy_st]

The l10n server rejects message submissions if the source message doesn't pass certain validation steps.

E.g. $key === generate_key($purified_source_message)
and $message === purify($message)

The output of purify() is slightly different e.g. if the message contains malformed HTML.

Right now, we catch such issues really late. After a release, when users start submitting translations.

And users will pick up a fix only by upgrading their source code (next G3 release).

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Fix:

Add a ("unit") test that uses the l10n_scanner and checks for each message that is found that:
HTMLPurifier::purify($message) === $message

(We'll need to package HTML purifier with the modules/gallery/test/.)