Add a download page for preview builds (#640)

- Make downloads data-driven, create a database of versions;
- Add a custom plugin to generate download URLs;
- Use the new plugin for the regular downloads page as well.

.editorconfig

@@ -11,3 +11,8 @@ trim_trailing_whitespace = true
 [*.{yml,yaml}]
 indent_style = space
 indent_size = 2
+
+# Ruby scripts and plugins for Jekyll
+[*.rb]
+indent_style = space
+indent_size = 2