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/markdown-editor/css/comments-list.css

@@ -3,9 +3,6 @@
   flex-direction: column;
   height: 100%;
 }
-.seafile-comment-list {
-  overflow-y: auto;
-}
 .seafile-comment-list .comment-vacant {
   padding: 1em;
   text-align: center;

frontend/src/pages/markdown-editor/css/side-panel.css

@@ -29,7 +29,7 @@
 }
 
 .seafile-markdown-editor .side-panel .nav .nav-link.active {
-  color: #f19645;
+  color: #ff9800;
   border-bottom: 0;
 }