Show cooldown info on accounts.

[td202]

Purpose

Closes #297

This supports cooldowns by updating how account information is shown to include cooldown status. Since it is permissible (from P7) to transition directly between baker and delegator, the required fields for configuring the baker and delegator are different in these cases (namely, you don't need to specify the stake and whether to restake).

Changes

• Display available balance returned by the node.
• Display the list of cooldowns on an account.
• When becoming a delegator, check that the necessary fields are provided.
• When becoming a validator, check that the necessary fields are provided.
• Update tests.

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.