Add sidebar option to define canonical link

Summary

Defining a canonical link will help admins to prevent duplicate content issues in search engine optimization by specifying the "canonical" or "preferred" version of a web page.

Requirements

• Add a sidebar section with the title "Search engine optimizations" (to potentiall add more options in the future).
• Inside that section add a url field titled "Canonical link".
• Add that sidebar section to all post types.
• If that urls is set add the relevant meta tag in that post header.
  <link rel="canonical" href="https://www.example.com/post/">

Resources

• Wikipedia page on canonical links