[feat] Feature flag system via config (#5960)

* [feat] Feature flag system via config

Adding a feature flag system that is driven by superset_config.py. This change includes:
- Server side changes to specify a dedicated FEATURE_FLAG dictionary for listing feature flags. E.g.
```
FEATURE_FLAGS = { 'SCOPED_FILTER': true }
```
- Pass the new feature flags to client via bootstrap-data
- Client side changes to inject feature flags into the redux state tree for dashboard, explore view and SqlLab
- Client side refactor/clean up so the feature flags can be properly tested. Also avoid modifying incoming bootstrap data when creating initial state for the redux state tree
- Re-enable tests that were previously disabled for ExploreViewContainer

* Fix lint errors.

* Remove the partial attempt to get reference to src working in tests (so we don't have to write ../../../src and such in tests). This will in a separate PR.

superset/assets/spec/javascripts/dashboard/containers/Dashboard_spec.jsx

@@ -0,0 +1,39 @@
+import React from 'react';
+import configureStore from 'redux-mock-store';
+import thunk from 'redux-thunk';
+import { shallow } from 'enzyme';
+import { expect } from 'chai';
+
+import Dashboard from '../../../../src/dashboard/containers/Dashboard';
+import getInitialState from '../../../../src/dashboard/reducers/getInitialState';
+
+describe('Dashboard Container', () => {
+  const middlewares = [thunk];
+  const mockStore = configureStore(middlewares);
+  let store;
+  let wrapper;
+
+  before(() => {
+    const bootstrapData = {
+      dashboard_data: {
+        slices: [],
+        metadata: {},
+      },
+      common: {
+        feature_flags: {
+          FOO_BAR: true,
+        },
+        conf: {},
+      },
+    };
+    store = mockStore(getInitialState(bootstrapData), {});
+  });
+
+  beforeEach(() => {
+    wrapper = shallow(<Dashboard />, { context: { store } });
+  });
+
+  it('should set feature flags', () => {
+    expect(wrapper.prop('isFeatureEnabled')('FOO_BAR')).to.equal(true);
+  });
+});