Design fixes (#555)

* Change placeholder color and design fixes for show page / sidebar

* Replace hardcoded border radiuses

* Improve border display for middle of button group

* Editor styling

* Editor font size

* Comment Bar positioning and remove scrollbar for 1px

* Add Comments section title

* Nit: match css style

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Co-authored-by: Emilien <emilien.chauvet.enpc@gmail.com>

front/src/modules/comments/components/right-drawer/RightDrawerTimeline.tsx

@@ -19,7 +19,7 @@ export function RightDrawerTimeline() {
 
   return (
     <RightDrawerPage>
-      <RightDrawerTopBar title="Timeline" />
+      <RightDrawerTopBar />
       <RightDrawerBody>
         {commentableEntityArray.map((commentableEntity) => (
           <Timeline