Remove dead package octicon

Octicon no longer provides its CSS classes and this way is useless in
CodiMD. Replacing all used classes in the UI and remove it from build
system.

Signed-off-by: Sheogorath <sheogorath@shivering-isles.com>

public/js/extra.js

@@ -834,7 +834,7 @@ const anchorForId = id => {
   const anchor = document.createElement('a')
   anchor.className = 'anchor hidden-xs'
   anchor.href = `#${id}`
-  anchor.innerHTML = '<span class="octicon octicon-link"></span>'
+  anchor.innerHTML = '<i class="fa fa-link"></i>'
   anchor.title = id
   return anchor
 }