Download Job: Truncate excessively long URLs

Limit them to a reasonable length, and don't send data URLs at all since they aren't human-readable and typically very long.

Modeled after what KIO does in a CopyJob.

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5.27 or 5.24, even, please

@fvogt @bf

Had to change the qMakePair<QString, QString> because I otherwise got

error: cannot bind rvalue reference of type ‘QString&&’ to lvalue of type ‘QString’