[FIX] mail: overflow in the attachment box

The chatter's attachment box has an horizontal overflow when adding at
least 1 attachment.

This is due to a negative margin applied on the attachment list/previews
not matching its container's padding (3px vs 5px).

This commit reuses the SCSS variable dedicated to this margin around the
attachment list/previews and apply it correctly to the chatter's
attachment box.

Task ID: 2194025

X-original-commit: 3a7eb3fe97541855ae893514f8b98cbfe6c1a7e4

addons/mail/static/src/scss/attachment_box.scss

@@ -4,7 +4,7 @@
         min-height: 50px;
 
         .o_chatter_attachment {
-            padding: 3px;
+            padding: $o-attachment-margin;
 
             input.o_input_file {
                 position: absolute;