refactor: clear up Plugin init, add activator/deactivator hooks

src/includes/class-footnotes-deactivator.php

@@ -0,0 +1,33 @@
+<?php
+/**
+ * Fired during plugin deactivation
+ *
+ * @since      2.8.0
+ *
+ * @package    footnotes
+ * @subpackage footnotes/includes
+ */
+
+/**
+ * Fired during plugin deactivation.
+ *
+ * This class defines all code necessary to run during the plugin's deactivation.
+ *
+ * @since      2.8.0
+ * @package    footnotes
+ * @subpackage footnotes/includes
+ */
+class Plugin_Name_Deactivator {
+
+	/**
+	 * Runs when the Plugin is deactivated.
+	 *
+	 * Currently NOP.
+	 *
+	 * @since    2.8.0
+	 */
+	public static function deactivate() {
+		// Nothing yet.
+	}
+
+}