feat(article): use markdownify for licence in article (#273)

* Use markdownify for licence in article

So we can use links, e.g. to credits for main photo.

* refactor: move .article-copyright style to partials/layout/article

Co-authored-by: Jimmy Cai <github@jimmycai.com>

layouts/partials/article/components/footer.html

@@ -4,7 +4,7 @@
     {{ if and (.Site.Params.article.license.enabled) (not (eq .Params.license false)) }}
     <section class="article-copyright">
         {{ partial "helper/icon" "copyright" }}
-        <span>{{ default .Site.Params.article.license.default .Params.license }}</span>
+        <span>{{ default .Site.Params.article.license.default .Params.license | markdownify }}</span>
     </section>
     {{ end }}
 
@@ -16,4 +16,4 @@
         </span>
     </section>
     {{- end -}}
-</footer>
+</footer>