Make popup wrapper position fixed

[beppek]

The popup wrapper position should be changed to fixed so the popup always appears at the right point for the user. Currently, when it's used in the investor list to view documents, if the user clicks to view a document further down the list the popup may appear partly, or wholly, outside of the screen.

If you don't agree with setting the position to fixed at least add a prop so this can be changed from outside. As far as I can tell I can't set this using the style prop as the Wrapper component which needs to have its position changed is not the outer most component in the Popup component.

Due to the impact this bug has on UX we should get this done asap.

[beppek]

It is the reason for this sale frontend issue which has a fairly high impact on merchant UX since they can't see the document modal in long lists when they click on investors further down the list.