chore: Enable "Substrate Dev" netwok in Dev mode

[Kriyszig]

Issue in focus: #402

• Removed in Comment out the Substrate Dev Stuff #384, this brings back the Substrate Dev network when __DEV__ is active

In case any changes are required, please let me know and I'll address them ASAP!

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[Kriyszig]

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[Tbaut]

Thanks for tackling this. It needs a little more than this if we want things to work.
There is also the need for adding the static-substrate.js because it's been deleted here https://github.com/paritytech/parity-signer/pull/384/files

Also as explained in the issue, the "Substrate Dev" should be shown in the Network chooser when we are in Dev.

Finally, the "Substrate Dev" Network should be uncommented in constants.js

[Kriyszig]

@Tbaut Made the necessary changes in the latest commit. Out if the commented lines I have replaced const isSubstrateDev = this.props.prefix === SUBSTRATE_NETWORK_LIST[SubstrateNetworkKeys.SUBSTRATE_DEV].prefix; with just __DEV__ to detect the Dev environment, should I revert that?

[Tbaut]

small grumbles, we're getting close :)

[Tbaut]

Thanks a lot, I've tested it with polkadot-js and it works well.