Add OpenCollective backers to foundation/sponsors

Author: wilzbach

foundation/sponsors.dd

@@ -41,8 +41,18 @@ $(UL
     $(LI C. Piker)
 )
 
+$(H2 Backers on OpenCollective)
+
+<script src='https://opencollective.com/dlang/banner.js?style={"a":{"color":"black"}}'></script>
 )
 
 Macros:
     TITLE=D Language Foundation Sponsors
     SUBNAV=$(SUBNAV_FOUNDATION)
+    EXTRA_HEADERS=$(T style,
+        #opencollective-banner h2 {
+          color: red;
+        }
+
+    )
+    _=