major: load Network from Context, enable adding Network from QR

[hanwencheng]

closes #302 closes #406

This PR enables adding new network / update existing network to Parity Signer via QR code

it Includes:

1. Save added / updated network into app storage.
2. Reafactor state management and use dynamic networks.
3. Add e2e test for adding new network and signing extrinsic on new network.
4. new tutorials on how to use this feature.

How to Test

1. Get the network metadata as QR Code

Switch to the network you want to play with on Polkadot.js app. Click Settings -> MetaData

Here we can see the chain specifications like Network Name, Address Prefix, and Genesis Hash etc. They are all the metaData of the network which is required by Parity Signer. The only item we could change is network color, it is used on Parity Signer to distinguish other networks.

On the right side is the QR Code we need.

2. Scan metadata QR code with Parity Signer

Now on the Parity Signer app, click the QR scanner Button anywhere on the app, and scan this QR code, you will have the new Network added to Parity Signer. You can now create accounts under it and sign extrinsic with this network.

Notice since the metadata is generally very big data, and currently, it is hard to sync with Parity Signer, so when signing the transactions on added networks, we cannot interpreter the extrinsic details at the moment. Please check on this issue for the update.