Add copyright headers and contributors list

Add proper copyright notices to all source files with accurate date
ranges per contributor based on git history. Add contributors to
composer.json authors and README.

ident_switch-form.js

@@ -1,5 +1,12 @@
-/*
- * This is part of identity_imap plugin
+/**
+ * ident_switch - Identity settings form handler.
+ *
+ * Copyright (C) 2018 Boris Gulay
+ *
+ * Original code licensed under GPL-3.0+.
+ * New contributions licensed under AGPL-3.0+.
+ *
+ * @url https://github.com/Gecka-apps/ident_switch
  */
 
 $(function() {