Add cyberzombie invasions in the nether and tweak zombie invasions

[saldejums]

Removes drop pod from zombies.
Gives zombies a slight speed buff and fixes hard invasions not applying the speed buff.
Adds cyberzombie invasions in the nether.

Numbers might need to be tweaked.

Resolves #928.

[saldejums]

Might be necessary to add a fire resistance potion effect to the cyberzombies because they can spawn in lava sometimes. Or edit the spawn rules but that might be more effort than it is worth.