Allow registration from NETWORK_PARENT_KEY

[HCastano]

In #918 (comment) and #955 (comment) there's been some discussions around
whether or not we should allow registration (and thus signing) from the network parent
key account.

Right now this account is blocked from registration, but I think we should allow it. By
doing so we would enable some cool use cases, like allowing governance to sign
transactions destined for foreign chains, like Bitcoin or Ethereum.

There may be other ways to achieve this, like having this network wide signature request
key be a child key like every other registered account.