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/pages/lib-content-view/lib-content-view.js

@@ -1819,7 +1819,6 @@ class LibContentView extends React.Component {
 
     return (
       <Fragment>
-        <div className="main-panel o-hidden">
           <div className="main-panel-north border-left-show">
             <LibContentToolbar
               isViewFile={this.state.isViewFile}
@@ -1947,7 +1946,6 @@ class LibContentView extends React.Component {
               />
             )}
           </div>
-        </div>
         {isCopyMoveProgressDialogShow && (
           <CopyMoveDirentProgressDialog
             type={this.state.asyncOperationType}