refactor: 💡 React components to consume appConfig using Context (#852)

* refactor: 💡 React components to consume appConfig using Context

React components to consume app configuration using React Context and
React Hooks. Non React components to continue using global variable
window.config. Related documentation also changed.

Closes: #725

* refactor: 💡 Removing unecessary code

On current React component there is no need to import useContext method
from React, so, removing it.

* refactor: 💡 Code review

Code review. Minor changes based on review inputs and moving userManager
to an init method

BREAKING CHANGE: #725

Closes: #725

* docs: don't include implementation detail in docs

* docs: don't include implementation detail in docs

* docs: no need to specify implementation details in employment recipe

* docs: no need to specify implementation details in deployment recipe

platform/viewer/src/context/AppContext.js

@@ -0,0 +1,5 @@
+import React from 'react';
+
+let AppContext = React.createContext({});
+
+export default AppContext;

platform/viewer/src/context/UserManagerContext.js

@@ -0,0 +1,5 @@
+import React from 'react';
+
+const UserManagerContext = React.createContext();
+
+export default UserManagerContext;

platform/viewer/src/context/WhiteLabellingContext.js

@@ -0,0 +1,10 @@
+import OHIFLogo from '../components/OHIFLogo/OHIFLogo.js';
+import React from 'react';
+
+const defaultContextValues = {
+  logoComponent: OHIFLogo(),
+};
+
+const WhiteLabellingContext = React.createContext(defaultContextValues);
+
+export default WhiteLabellingContext;