Refactor lockup to use master account

[ilblackdragon]

The suggestion is to change lockup to have 0 access keys.

Instead have a master account that is specified on deployment.

• Lockup contract is only aware of it’s master account
• master account can have a multisig instead of full access key to improve security and allow key management
• master account can authorize staking UI for using lockup contract on it’s behalf of it with stake,deposit,withdraw,unstake authorized functions.

Meaning that all public methods inside lockup for user just have a guard of assert_eq!(env::predessesor(), self.owner_id); and methods for foundation have assert_eq!(env::predessesor(), self.foundation_id);

On the wallets / custody side, this would mean that they can just control master account. A separate page for lockup operations can be created, where users can "Login with NEAR" and then withdraw vested tokens. Staking of locked tokens can be done from this page as well (or separate staking app can be used, but needs to be aware of this dynamics).

We can create master accounts via linkdrops with multisig contract as default and only enough $NEAR to cover the contract storage + 3 keys.

CC @kcole16