MM-11036: subpath eliminate hardcoded /static/ (mattermost#1389)

* include window.basename when resolving emojis via the EmojiStore

* load img_trans.gif via webpack to automatically point into subpath

* use webpack-pwa-manifest to manage manifest.json

This plugin also handled injecting <link rel="apple-touch-icon" ... />
and <meta name="apple-*" ... /> tags into root.html.

components/emoji_picker/components/emoji_picker_item.jsx

@@ -6,6 +6,7 @@ import React from 'react';
 import debounce from 'lodash/debounce';
 
 import EmojiStore from 'stores/emoji_store.jsx';
+import imgTrans from 'images/img_trans.gif';
 
 const SCROLLING_ADDITIONAL_VISUAL_SPACING = 10; // to make give the emoji some visual 'breathing room'
 const EMOJI_LAZY_LOAD_SCROLL_THROTTLE = 150;
@@ -74,7 +75,7 @@ export default class EmojiPickerItem extends React.Component {
  image = (
  <img
  onMouseOver={this.handleMouseOverThrottle}
- src='/static/images/img_trans.gif'
+ src={imgTrans}
  className={spriteClassName}
  onClick={this.handleClick}
  />

components/emoji_picker/components/emoji_picker_preview.jsx

@@ -6,6 +6,7 @@ import React from 'react';
 import {FormattedMessage} from 'react-intl';
 
 import EmojiStore from 'stores/emoji_store.jsx';
+import imgTrans from 'images/img_trans.gif';
 
 export default class EmojiPickerPreview extends React.Component {
  static propTypes = {
@@ -28,7 +29,7 @@ export default class EmojiPickerPreview extends React.Component {
  previewImage = (
  <span className='sprite-preview'>
  <img
- src='/static/images/img_trans.gif'
+ src={imgTrans}
  className={'emojisprite-preview emoji-category-' + emoji.category + '-' + emoji.batch + ' emoji-' + emoji.filename}
  />
  </span>

components/emoji_picker/emoji_picker.jsx

@@ -8,6 +8,7 @@ import throttle from 'lodash/throttle';
 import EmojiStore from 'stores/emoji_store.jsx';
 import * as Emoji from 'utils/emoji.jsx';
 import * as Utils from 'utils/utils.jsx';
+import imgTrans from 'images/img_trans.gif';
 
 import EmojiPickerCategory from './components/emoji_picker_category';
 import EmojiPickerItem from './components/emoji_picker_item';
@@ -530,7 +531,7 @@ export default class EmojiPicker extends React.PureComponent {
  ref={ref}
  >
  <img
- src='/static/images/img_trans.gif'
+ src={imgTrans}
  className='emojisprite'
  />
  </div>