Delete lockup UI Button

[corydickson]

Allow users to delete their lockup returning the storage deposit. For an initial version of this feature, the user's lockup should have no NEAR balance greater than 2.1 in the lockup, no pending balance, and no LSTs deposited in the lockup.

Once this target contract state is reached, display a button in the manage assets page.

In progress PR: houseofstake/agora-near#241

Happy Path

• Users have only one LST in their lockup and have staked through the UI
• Call delete_lockup on lockup contract once the balance checks above have been satisfied

Unhappy Path

• Users have more than one LST in the lockup See Manage unstaking Tokens / Withdraw UI product#45
• Until we support unstaking for multiple LSTs the will not be able to get the lockup contract into the target state above through the UI. Instead they will need to do this manually to see the button

[jefag]

Explicitly, we aren't handling the unhappy path here (ie, we're not handling multiple LSTs).