Plugin for checking of contract expiration

Some of the first Turris routers' contracts are going to expire soon. We need a plugin that can answer if that already happened or not. The web API is ready.

If it didn't expire, it should also set the corresponding user lists in its post-inst script.

• Plugin to check for contract expiration
• Script to auto-enable corresponding user list.
• A cron script and postinst script.
• A configuration flag into the updater config to re-plan after installation of nuci (because it might have updated the user lists).