Remove network switching

[kieranhall]

We will publicly serve the explorer on three separate sub domains:

• devnet.apps.dusk.network/explorer
• testnet.apps.dusk.network/explorer
• apps.dusk.network/explorer

These for the 3 networks (devnet, testnet and mainnet).

1. Change the explorer so that it doesn’t use network requests for graphQL, because that will all be called on /
2. Remove the network dropdown code.
3. Use an env var to add an optional port to add to the local URL used for API calls. (This is to be flexible when developing the app.)
4. We need a proxy on apps.dusk.network/explorer pointing to a node (presumably on port 80?) (so this would be replacing the current component on DO with a proxy). We also need this to work for devnet.* and testnet.* domains to point to explorers on those networks.
5. Configuration so that Rust bundles the explorer and that there can be env variable configuration for the node to serve the explorer or not.