Seahub UI (#5261)

* [top toolbar] fixed the position of the vertical line between 2 sides

- fixed it for all the pages
- removed the duplicated 'div.main-panel' in 'lib-content-view'

* [search] modified the UI

* [top toolbar] redesigned the notification

* [top toolbar] avatar & the triangle: modified the UI

* [left side panel] updated the UI; fixup & improvement

* [misc] modified links' color, op-icon, and etc.

* [misc] updated UI for share dialog, other dialogs, & 'more operations' dropdown

* [misc] UI: modified markdown side panels, comment panels, input/textarea placeholder

* ['user settings' page] modified UI

* [shared dir upload link page] modified the UI

* [user settings] modified the UI

* [user settings] fixup

* [seafile-ui.css] use an updated one (modified color of buttons)

frontend/src/components/main-panel.js

@@ -9,7 +9,7 @@ class MainPanel extends Component {
 
   render() {
     return (
-      <div className="main-panel o-hidden">
+      <div className="main-panel">
         {this.props.children}
       </div>
     );