Add statistic tracking

database/migrations/06_add_last_shared_at_to_users_table.sql

@@ -0,0 +1 @@
+ALTER TABLE users ADD last_shared_at DATETIME;

database/migrations/07_create_statistics_table.sql

@@ -0,0 +1,9 @@
+CREATE TABLE IF NOT EXISTS statistics (
+     id INTEGER PRIMARY KEY AUTOINCREMENT,
+     timestamp DATE,
+     shared_sites INTEGER,
+     shared_ports INTEGER,
+     unique_shared_sites INTEGER,
+     unique_shared_ports INTEGER,
+     incoming_requests INTEGER
+)