seen: Improve readability of information

[kwaaak]

Now that #1560 has been merged, let's use it.

This changes the reply to "seen" commands:
before
nickname: I last saw other_nickname at 2019-05-29 - 23:04:13UTC in here, saying gotta go
after
nickname: I last saw other_nickname in here 45 days, 4 hours ago, saying: gotta go

I don't know if the timestamp should be included in the reply or not.

[Exirel]

Looks like a good improvement to me. 👍

[kwaaak]

This change drastically simplifies the module.

Instead of doing
bot.db.set_nick_value(trigger.nick, 'seen_timestamp', time.time())
we can just do
bot.db.set_nick_value(trigger.nick, 'seen_timestamp', trigger.time)
and get rid of the datetime and time imports.

The code in this module runs a lot. The datetime object trigger.time would be twice as big as the float time.time().

[Exirel]

🚢