Refactor the TokenPicker component

[six7]

Our current TokenPicker component is quite poor when it comes to UX.

It wasn't implemented correctly as it's not using a Portal, and as such is always bound by to the constraints of the parent.

We worked around this by increasing the parent's height (as can be seen in the Inspect/Remap modal), but ideally this should just use a Portal and live outside of the parent container.

What I'd like us to do is refactor this so it can use a Portal.