Don't accept sandbox attribute
Don't accept sandbox attribute

Because sandbox is whitelist attribute, attacker will be able to create iframe that has more permission than default.

Signed-off-by: RyotaK <c551b9cb05aa2c497697187c2e85a0272d201bde@users.noreply.github.com>

hackmdio/hackmd,hackmdio/hackmd,jackycute/HackMD,jackycute/HackMD,hackmdio/hackmd,jackycute/HackMD