Add Safe Mode pallet to runtime

[keithtensor]

We would like to add the Safe Mode pallet to our subtensor runtime. This pallet allows a highly privileged user such as root to put the entire chain into "safe mode" for a set period of time, whereby only a select few extrinsics can be executed, while all others will be rejected. While in safe mode, the privileged user can also choose to extend the duration of the safe mode period.

Acceptance Criteria

• Figure out and configure who should have the authority to put the chain into and out of safe mode (root)
• Figure out and configure how long safe mode can last
• Figure out and configure how long an extension can last
• Add the configured pallet to the runtime