Allow explicit removal of assets

It is desirable to simply remove an asset from the portfolio entirely. It might be best to simply just filter out assets with a value of 0 from the portfolio list, and then add an option to show all assets with a 0 value that have been previously held.

It would be nice to add a button with confirmation that simply zeroes out an asset in a couple of clicks where you have some small amount remaining for an asset, but you just want to forget about it.