Add beta network to dfx.json

[dskloetd]

Motivation

We are currently using the app network canister for the beta release.
We want to move the beta release to Orbit for more secure management and make the app canister available of ad-hoc testing again.

Changes

1. Add a beta network to dfx.json which has all the same values as mainnet and app except for the nns-dapp canister ID, which is the one controlled by the NNS team Orbit station.
2. Set IS_TEST_MAINNET to true for the beta network in addition to the app network so that we show the warning banner.

Tests

1. Manually tested that the warning banner is show when the network is set to beta or app.
2. Ran DFX_NETWORK=beta ./config.sh and inspected that the arguments are as described here.

Todos

• Add entry to changelog (if necessary).
  not necessary