Change potion time from ticks to seconds

[ncurran02]

Suggested by someone on Discord - change the time in potion command from ticks to seconds.

[characterslimits]

Actually, I would like to propose instead of completely changing time from ticks to seconds, instead add time unit support.

What does this mean? Basically if you enter "30s" as the time in /ii potion add SPEED 30s 0 it should interperate that as "30 seconds". The same as "2m" for 2 minutes, "5h", and so on...

If you want to still use ticks you can use "300t" for 300 ticks.

The default should remain ticks if a time unit is not specified for compatilibity reasons.