Replace Lockdown Mode pallet by FRAME SafeMode & TxPause pallets

[stiiifff]

Trappist runtime includes a custom built Lockdown Mode pallet that provides the ability to temporarily pause transfers & normal operations on a live parachain, which is a valuable mechanism to have at the team’s disposal when an issue / attack occurs.
The Substrate FRAME library now contains the SafeMode & TxPause pallets which allow implementation of such a mechanism.

We should prefer the new SafeMode pallet over Trappist’s custom-built one as the former has been more extensively reviewed and audited, and thus, replace the Lockdown Mode pallet by the FRAME SafeMode & TxPause pallets.

The Lockdown Mode pallet should be removed from the Trappist repository, and moved to a separate one, and archived.

• Integrate the FRAME SafeMode & TxPause pallets with sane defaults in both devnet & mainnet runtimes (disabled by default in both runtimes).
• SafeMode should only allow balance transfers as whitelisted call. Permissionless operations (enter, extend) should be disabled. Only a privileged origin can enable / disable safe mode: ForceEnterOrigin, ForceExtendOrigin & ForceExitOrigin to be configured as the Root origin (Sudo).
• TxPause PauseOrigin & UnpauseOrigin to be configured as the Root origin (Sudo).
• Remove the Lockdown Mode pallet from the Trappist repository (move it to a separate new repository, and mark it as archived).

Depends on #278

[Moliholy]

I just added #305 to address this issue.

I also created https://github.com/Moliholy/pallet-lockdown-mode under my GitHub to transfer its ownership to Parity whenever we want.