chore: update nav & footer links

_config.icarus.yml

@@ -79,6 +79,9 @@ navbar:
         About: /about
     # Links to be shown on the right of the navigation bar
     links:
+        Join Gitter:
+            icon: fab fa-gitter
+            url: 'https://gitter.im/hexo-theme-icarus/community'
         Download on GitHub:
             icon: fab fa-github
             url: 'https://github.com/ppoffice/hexo-theme-icarus'
@@ -89,9 +92,9 @@ footer:
         Creative Commons:
             icon: fab fa-creative-commons
             url: 'https://creativecommons.org/'
-        Attribution 4.0 International:
-            icon: fab fa-creative-commons-by
-            url: 'https://creativecommons.org/licenses/by/4.0/'
+        Join Gitter:
+            icon: fab fa-gitter
+            url: 'https://gitter.im/hexo-theme-icarus/community'
         Download on GitHub:
             icon: fab fa-github
             url: 'https://github.com/ppoffice/hexo-theme-icarus'

source/_posts/en/Upgrade-Guide.md

@@ -67,6 +67,6 @@ toc: true
    You can still set `thumbnail` in the front-matter to show a smaller thumbnail image for your posts 
    in the archive page and in the recent post widget.
 
-6. Check out the [release notes](https://github.com/ppoffice/hexo-theme-icarus/releases) and the 
+6. Check out the [release notes](https://github.com/ppoffice/hexo-theme-icarus/releases) and the
    [latest documentation](https://ppoffice.github.io/hexo-theme-icarus/categories/) for more information 
    of the new features.

source/_posts/zh-CN/Upgrade-Guide.md

@@ -68,5 +68,4 @@ toc: true
 
    你仍可以在front-matter中设置`thumbnail`来在文章归档页面以及最新文章挂件中显示小一点的缩略图。
 
-6. 阅读[更新说明](https://github.com/ppoffice/hexo-theme-icarus/releases)和
-   [最新文档](https://ppoffice.github.io/hexo-theme-icarus/categories/)来了解更多新功能的信息.
+6. 阅读[更新说明](https://github.com/ppoffice/hexo-theme-icarus/releases)和[最新文档](https://ppoffice.github.io/hexo-theme-icarus/categories/)来了解更多新功能的信息.