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/css/toolbar.css

@@ -11,7 +11,7 @@
 .border-left-show:before {
   position: absolute;
   top: 10px;
-  left: 0;
+  left: -1px;
   width: 1px;
   height: 30px;
   content: '';
@@ -79,6 +79,7 @@
 .common-toolbar {
   display: flex;
   margin-left: auto;
+  align-items: center;
 }
 
 .common-toolbar > div{