Channel switcher UI improvements (#6564)

* Channel switcher UI improvements

* Reverting mattermost theme

* Fixing spacing above the button

* Updating switcher button

* PLT-6691 - Fixing tablet header

webapp/components/quick_switch_modal/quick_switch_modal.jsx

@@ -301,7 +301,7 @@ export default class QuickSwitchModal extends React.PureComponent {
                 <Modal.Header closeButton={true}/>
                 <Modal.Body>
                     {header}
-                    <div className='modal__hint'>
+                    <div className='modal__hint hidden-xs'>
                         {help}
                     </div>
                     <SuggestionBox