Check tag exists before linking

[KyleMit]

Make sure the tag name exists before hyperlinking to it

We currently do this for authors in layouts/post.njk:

{% set authorInfo = collections.authors[author] %}
{% if authorInfo %}
  <a href="/pages/authors/{{author | lower}}">{{ author }}</a>
{% else %}
  {{ author }}
{% endif %}

We could probably do the same for tags as we loop through before making them all hyperlinkable

{% for tag in tags | contentTags %}
  <li class="tag">
    <a href="/tags/{{tag}}" class="tag-badge">{{ tag }}</a>
  </li>
{% endfor %}