* Make App.js into skeleton for GraphExplorer
We make a very basic skeleton for the Graph Explorer as a basis
for future development.
This commit also removes the UserExplorer and FileExplorer from
App.js. Since we have changed the underlying data model, we are
unlikely to use the UserExplorer or FileExplorer in anything like
their current state, so they are effectively deprecated. I am deferring
removing them because it is nice to have some examples of working React
code to copy from, before the Graph Explorer is ready.
Test plan: run `yarn start`, and observe that the App displays the
words "Graph Explorer" underneath the "SourceCred Explorer" title bar.
Reorganize the code so that we have a single package.json file, which is at the root.
All source code now lives under `src`, separated into `src/backend` and `src/explorer`.
Test plan:
- run `yarn start` - it works
- run `yarn test` - it finds the tests (all in src/explorer) and they pass
- run `yarn flow` - it works. (tested with an error, that works too)
- run `yarn prettify` - it finds all the js files and writes to them
