fix: hide keyline when footnotes are preceded by header (#4529)

Hide key-line created by `markdown-it-footnote` template when footnotes are directly preceded by header.

Examples:
```md
# References
[^1]: Shows no key-line because References header has one already

# New Page
Text goes here and whatever else

[^1]: these footnotes have no preceding header and will show a key-line
```

client/themes/default/scss/app.scss

@@ -77,6 +77,10 @@
     & + h2, & + h3, & + h4, & + h5, & + h6 {
       margin-top: 8px;
     }
+
+    & + hr.footnotes-sep {
+      display: none;
+    }
   }
 
   h1 {