Fix Stake CAP approval pending state

[yangyu0330]

Closes #32.

Summary

• Check CAP allowance against the Staking contract, which is the contract used by depositCAP() for staking.
• Keep the Stake CAP modal in a loading state while allowance is being checked or the approval transaction is pending.
• Prevent the Stake action from submitting while approval is still required or allowance is still loading.

Verification

• npm run build

I also requested assignment in the issue for bounty eligibility.