Fix confirm token transaction amount display

[Gudahtt]

The token amount displayed when confirming a token transaction was wrongly being converted to a string in the container. As a result, the conversion into the user's preferred currency would fail.

A default value of '0' was added for the token amount as well, to prevent undefined from being rendered as the value. Really the value should never be undefined, but it was rather difficult to handle that case without a deeper investigation into how it might occur. The 0 default is consistent with existing rendering logic.

[danjm]

This looks good to fix the immediate issue.

An improvement that should be made at some point is to move the calculation of the amount in the user's preferred currency into the container instead of doing it within a component method. This applies to multiple calculations done in the component however, so I think it's okay to skip such changes for this PR... in fact this is really an unrelated code structure/organization point. I suppose I bring it up because an alternative way to fix this would be to ensure that methods that operate on this value can accept numbers in string or number type (as BN.js does, for instance), but this is another separate discussion.

[Gudahtt]

Agreed - the current situation leaves much to be desired.

I would also like to move things around to ensure that it's impossible for the token amount to be undefined. The effort that would require seems daunting though. Maybe that could be done as part of an effort to move the state management for confirming transactions into a "ducks" style module.