Remove non-null assertion in ConversationProvider

- Add `createOptionalContext` that is used by `AuthContext` and `ConversationContext` to create a context with a hook
  that can be used to retrieve its value and throw an error if it's undefined.
- In `router.tsx`, put `Messenger` inside `ConversationProvider`.
- In `ConversationListItem`, use the conversationId from the `ConversationContext ` instead of the url params.
- Fix bug in `CallInterface` with fullscreen.
- Remove unecessary useEffect dependency in `NotificationManager`.

Change-Id: Ib5f0ae6a0a34cdbdb02f871e36194376d945230d
diff --git a/client/src/contexts/ConversationProvider.tsx b/client/src/contexts/ConversationProvider.tsx
index 25137c0..b927278 100644
--- a/client/src/contexts/ConversationProvider.tsx
+++ b/client/src/contexts/ConversationProvider.tsx
@@ -16,9 +16,10 @@
  * <https://www.gnu.org/licenses/>.
  */
 import { ConversationView, WebSocketMessageType } from 'jami-web-common';
-import { createContext, useContext, useEffect, useState } from 'react';
+import { useContext, useEffect, useState } from 'react';
 
 import LoadingPage from '../components/Loading';
+import { createOptionalContext } from '../hooks/createOptionalContext';
 import { useUrlParams } from '../hooks/useUrlParams';
 import { Conversation } from '../models/Conversation';
 import { ConversationRouteParams } from '../router';
@@ -27,12 +28,14 @@
 import { useAuthContext } from './AuthProvider';
 import { WebSocketContext } from './WebSocketProvider';
 
-interface IConversationProvider {
+interface IConversationContext {
   conversationId: string;
   conversation: Conversation;
 }
 
-export const ConversationContext = createContext<IConversationProvider>(undefined!);
+const optionalConversationContext = createOptionalContext<IConversationContext>('ConversationContext');
+const ConversationContext = optionalConversationContext.Context;
+export const useConversationContext = optionalConversationContext.useOptionalContext;
 
 export default ({ children }: WithChildren) => {
   const {