Add tracking for time since last login

[mheguy]

Essentially this will be to demote regulars who haven't been on in multiple months. Lately we had former regulars come on and grief, so this is in the hopes of reducing occurrences like that.

Time since last logon will need some server-side bits since we can't get the system time from inside the game.

Quoting @grilledham from a former thread:

I'm working on a way to get server time from a script. Should open up some possibilities.

Related: #472

[theorangeangle]

Somewhat related: When 0.17 hits maybe we should just purge the regulars list and start fresh with a higher requirement for persistent regular rank (since the regular permissions are given after a few hours anyway automatically, just not the rank IIRC) if playtime can be totaled.

[linaori]

I believe we already have access to the server time

[linaori]

Additional stat tracking server side that might be interesting, is to also store how much time per map is played. This will let us see which maps are very popular.

[mheguy]

This is great timing, thanks lin! Playtime per player and total playtime are two great stats to track on maps.