Added an oauth dropbox page

pages/oauth/dropbox.html

@@ -0,0 +1,15 @@
+---
+layout: oauth
+title: Greenshot authenticated with Dropbox
+permalink: /oauth/dropbox
+categories: [oauth]
+tags: [uses_alerts]
+comments: []
+---
+{% include oauth-out-of-bounds.html %}
+{% include version-check.html %}
+<p>
+Greenshot should now be authenticated to use Dropbox, you can close this window.
+<p>
+<strong>We are happy that you use Greenshot. If you like it, please consider to support the development of this open source screenshot tool with a donation.</strong></p>
+{% include donate.html %}