Confirm navigation when it would discard unsaved changes

- New state field: `state.entryDraft.hasChanged`, initialized to
  `false`.

- `state.entryDraft.hasChanged` set to `true` in `entryDraft` reducer
  for `DRAFT_CHANGE_FIELD`.

- `EntryPage` adds a `listenBefore` listener to `history` on
  `componentDidMount` that checks `this.props.entryDraft.hasChanged`
  and, if that is true, asks the user to confirm the navigation.

- `EntryPage` removes its listener on `componentWillUnmount`.

src/reducers/entryDraft.js

@@ -19,7 +19,13 @@ import {
   REMOVE_ASSET,
 } from '../actions/media';
 
-const initialState = Map({ entry: Map(), mediaFiles: List(), fieldsMetaData: Map(), fieldsErrors: Map() });
+const initialState = Map({
+  entry: Map(),
+  mediaFiles: List(),
+  fieldsMetaData: Map(),
+  fieldsErrors: Map(),
+  hasChanged: false,
+});
 
 const entryDraftReducer = (state = Map(), action) => {
   switch (action.type) {
@@ -47,6 +53,7 @@ const entryDraftReducer = (state = Map(), action) => {
       return state.withMutations((state) => {
         state.setIn(['entry', 'data', action.payload.field], action.payload.value);
         state.mergeIn(['fieldsMetaData'], fromJS(action.payload.metadata));
+        state.set('hasChanged', true);
       });
     
     case DRAFT_VALIDATION_ERRORS: