Moving dev dependencies to dev, removing unused deps, updates some of them

Summary:
Moves dependencies only required to development to devDependencies. Removes artifact dependencies. Updates some of them.

List of updated modules (to latest version):

babel-plugin-named-asset-import
camelcase
dotenv
lint-staged
prettier
sass-loader
css-loader

The previous issue with css-loader breaking npm start appears to have been resolved by the resolution of the React 16 conflict in earlier diffs and updates to other css dependencies.

Test Plan:

npm start

Observe app loads correctly, no errors in console
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Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8905