Add copy button on "Export private key" modal

We should add a "copy" button next to the private key on the "Export private key" modal. Currently the key can be copied by clicking on the private key, but this deviates from the pattern we use elsewhere in the extension and is not keyboard accessible.

We should also consider placing a copy button alongside the account address. I am unsure whether that is necessary here though.

Start by locating the Export private key modal and the existing copy-button pattern elsewhere in the extension. Add a keyboard-accessible copy button beside the private key, then verify whether the account address also needs one based on the surrounding UI pattern.

Written by the indexing model from the issue text.