Advertise Infclass client

[Kaffeine]

On a player connected the server but at least 24 hours later after the prev message send a one-line message (as a part of 'welcome'):

• Inform "not-infclass" client users about the client existance
• Remind old Infclass client users to update

Sample text:

1. "Try official Infclass client for better UX on infclass servers. Visit https://infclass.github.io for details"
2. "You're using an outdated Infclass client version. Visit https://infclass.github.io for details"

Add new config options:

• inf_client_ad_interval (1440, 0, 99999)
• inf_client_current_version (180, 0, 9999)
• about_client_url (128, "https://infclass.github.io")

Unresolved question: how to store the list of advertised players with timestamps. Store = "across server restarts".