feat: add rel-me and mastodon (#114)

![preview](https://gitea.com/attachments/6aa08248-ca59-4f7c-82e2-97ab6d5984a3)

Closes #106

Co-authored-by: jolheiser <john.olheiser@gmail.com>
Reviewed-on: https://gitea.com/gitea/website/pulls/114
Reviewed-by: Lauris BH <lafriks@noreply.gitea.io>
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: John Olheiser <john+gitea@jolheiser.com>
Co-committed-by: John Olheiser <john+gitea@jolheiser.com>

config.yaml

@@ -64,6 +64,12 @@ menu:
       url: https://discourse.gitea.io/
       weight: 70
       pre: group
+    - name: Mastodon
+      url: https://social.gitea.io/@gitea
+      weight: 75
+      pre: mastodon
+      params:
+        rel: me
     - name: Twitter
       url: https://twitter.com/giteaio
       weight: 80

layouts/partials/toplinks.html

@@ -14,7 +14,7 @@
 			</div>
 		</div>
 	{{ else }}
-		<a class="navbar-item" href="{{ .URL }}">
+		<a class="navbar-item" {{ if eq .Name "Mastodon" }}rel="me"{{ end }} href="{{ .URL }}">
 			<i class="fa fa-{{ .Pre }}"></i>&nbsp;
 			{{ .Name }}
 		</a>