Externally connected SSDs

[jgresham]

Feature details

Right now NiceNode allows a user to run a node and select an externally connected device for the node's storage. However, there are a few changes needed.

1. NN should scan for externally connected SSDs before showing storage warnings. Right now it just looks at storage which the OS is installed on.
2. When a user is adding a node and the Operating System storage does not have enough storage, NN should prompt a user to use an external SSD, if an external SSD is connected.
3. NN should also suggest external SSDs as an option in Hardware Requirements in the app and on the docs website. This is going to be a very appealing option for our targeted users (those who don't want to open their computer to add storage). Also external SSDs are a great option for those who might move their node between computers.