PLT-4602 Fix misaligned close button and fix Android icon rendering bug (#4492)

* PLT-4602 Fix horizontal alignment for .close__button

Reduce negative left margin to fix miscalculated center-positioning.

* PLT-4602 Fix dropdown close icon rendering bug on Android

The combination of using a unicode character and a pseudo element was
too problematic, so I added a new custom icon svg CLOSE_ICON that
renders consistently on all browsers.

* PLT-4602 Use font-family fix, remove <svg>

You were correct @hmhealey, the problem was that the unicode characters
were rendering as different fonts: Open Sans and Helvetica Neue!

webapp/components/navbar.jsx

@@ -546,6 +546,12 @@ export default class Navbar extends React.Component {
                             {deleteChannelOption}
                             {leaveChannelOption}
                             {toggleFavoriteOption}
+                            <div
+                                className='close visible-xs-block'
+                                onClick={() => this.refs.headerOverlay.hide()}
+                            >
+                                {'×'}
+                            </div>
                         </ul>
                     </div>
                 </div>
@@ -671,7 +677,7 @@ export default class Navbar extends React.Component {
                         options={{singleline: true, mentionHighlight: false}}
                     />
                     <div
-                        className='close__icon visible-xs-block'
+                        className='close visible-xs-block'
                         onClick={() => this.refs.headerOverlay.hide()}
                     >
                         {'×'}
@@ -721,7 +727,7 @@ export default class Navbar extends React.Component {
                             />
                         </div>
                         <div
-                            className='close__icon visible-xs-block'
+                            className='close visible-xs-block'
                             onClick={() => this.refs.headerOverlay.hide()}
                         >
                             {'×'}