[Wallet] Allow arbitrary assets

[thomaseizinger]

trafficstars

At the moment our wallet understands two different assets: L-USDT and L-BTC (sometimes referred to as the native asset). The according asset IDs are configured in the browser's localstorage and can be changed through the extension's option page our through setting certain environment variables during compile time.

There are a few limitations here:

1. Unknown assets are not detected by the wallet
2. If an asset is wrongly configured it is not shown (as it is basically unknown)
3. Bobtimus might have a different opinion on asset Ids

This leads to some random errors where the wallet cannot participate in a swap because it does not know about the assets Ids.

Ideally, the wallet knows about all assets for which it owns the key to.