Move roomnameGenerator.js out of features/base/util

Over time features/base/util became a bucket where people seemed to dump
just about anything they couldn't think of a better place for. That's a
trend I don't like encouraging. Given that roomnameGenerator.js is
currently used in features/welcome only, I'm fine with moving it there
for the greater good.

react/features/base/util/index.js

 export * from './loadScript';
 export * from './randomUtil';
-export * from './roomnameGenerator';

react/features/welcome/components/AbstractWelcomePage.js

 import { isRoomValid } from '../../base/conference';
 import { VideoTrack } from '../../base/media';
 import { getLocalVideoTrack } from '../../base/tracks';
-import { generateRoomWithoutSeparator } from '../../base/util';
+
+import { generateRoomWithoutSeparator } from '../roomnameGenerator';
 
 /**
  * Base (abstract) class for container component rendering the welcome page.

react/features/base/util/roomnameGenerator.js → react/features/welcome/roomnameGenerator.js

 /* @flow */
 
-import { randomElement } from './randomUtil';
+import { randomElement } from '../base/util';
 
 /*
 const _NOUN_ = [

react/features/welcome/route.js

 /* global APP */
 
 import { RouteRegistry } from '../base/react';
-import { generateRoomWithoutSeparator } from '../base/util';
 
 import { WelcomePage } from './components';
+import { generateRoomWithoutSeparator } from './roomnameGenerator';
 
 /**
  * Register route for WelcomePage.