Add an adjusted value calculator

## Problem

Routes like `/portfolio/ABC` show a page displaying all available information for a given currency in a portfolio. There is a large amount of space available on the page. One regular task I find myself doing in the Python shell is working out how much money I will get if I sell some currency at different prices.

## Solution

It would be good to add an input to the page which is auto-filled with the current price of an asset, which can be updated to reflect a different price for that asset, and which will display the total value of the asset for that adjusted price. (`Amount * NewRate`).