Support restoring an existing wallet via seed or keys

[bitlamas]

Support for restoring a wallet that was created in another device is a core functionality that is currently missing on the Edge app. This feature can be seen in most wallets and could be also reproduced on Edge.

For Monero, specifically, the process should ask for the seed (or keys), a block height for when to start syncing or simply a date. I find that offering a date is pretty straightforward for most users. Since timezones are a factor, I'd recommend scanning the blockchain from the date the user selected, minus 7 days -- this way you're sure not to skip any blocks that might contain a transaction.