Make transactions human-readable

[frol]

Currently, Explorer only shows the raw data, but to make this tool useful for humans, we should feature human-readable explanations, e.g. "New Account + Add Key + Transfer" = "User X created a new account Y and funded it with N tokens. X has no control over Y, and only the newly added key ed25519:... can initiate transactions on behalf of Y account"

This was inspired by several conversations with various stakeholders: @reefoh @ilblackdragon @mikedotexe, so we even consider to make it a separate JS package, so it can be reusable outside of Explorer (the API is not clear to me now, but we can sync with @mikedotexe about that later)