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/comment-thread/CommentThreadBodyEditor.tsx

@@ -46,7 +46,7 @@ export function CommentThreadBodyEditor({ commentThread, onChange }: OwnProps) {
     initialContent: commentThread.body
       ? JSON.parse(commentThread.body)
       : undefined,
-    editorDOMAttributes: { class: 'editor-edit-mode' },
+    editorDOMAttributes: { class: 'editor' },
     onEditorContentChange: (editor) => {
       debounceOnChange(JSON.stringify(editor.topLevelBlocks) ?? '');
     },