feat: Support Roundcube 1.7 static files handling

Roundcube 1.7 forces us to go trough static.php for security. However,
this script does not exist in older Roundcube releases. To maintain
backwards compatibility, figure out the correct path at runtime
depending on the version.

Author: GermanCoding

README.md

@@ -25,7 +25,7 @@ The plugin name to add to your config file is: `tls_icon`.
 
 ## Requirements
 
-- Roundcube `1.3.0` or newer.
+- Roundcube `1.3.0` or newer (tested up to 1.7).
 - PHP `5.4` or newer.
 
 ## Currently supported languages