Change the filter link buttons

When disabled they are link icons; when enabled they are either
link (bottom) or offset (top) icons

Closes surge-synthesizer#2286

README.md

@@ -268,4 +268,5 @@ Azure pipelines for free to the open source community!
 
   * Most Surge-related conversation on the Surge Synth Slack. [You can join via this link](https://raw.githubusercontent.com/surge-synthesizer/surge-synthesizer.github.io/master/_includes/slack_invite_link)
   * IRC channel at #surgesynth at irc.freenode.net. The logs are available at https://freenode.logbot.info/surgesynth/.
-  * Discussion at KvR forum [here](https://www.kvraudio.com/forum/viewtopic.php?f=1&t=511922)
+  * Discussion at KvR forum [here](https://www.kvraudio.com/forum/viewtopic.php?f=1&t=511922)
+  * Chain icon on filter link from flaticon.com