monthlyAmount prop in WalletPanel should be type string

[ryanml]

Related: brave/brave-core#957

Should be string amount formatted like "5.0"

Current number typing is causing issues, passing in '5' (number) will not match any of the float token values. This is broken at this time and does not work even in storybook.

Per design we will soon need to have a string option in the wallet panel dropdown, such as 'Off'

This is in line with the typing for value on <Select />