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/ui/components/form/Checkbox.tsx

@@ -25,7 +25,7 @@ const StyledContainer = styled.div`
 
   input[type='checkbox']::before {
     border: 1px solid ${({ theme }) => theme.font.color.tertiary};
-    border-radius: 2px;
+    border-radius: ${({ theme }) => theme.border.radius.xs};
     content: '';
     display: block;
     height: 12px;