Resistance Buffs & Rings balancing

To the delight of mages and the cries of the masses, this pull request
severely nerfs resistance buffs. There are two reasons for this:

1 - New systems of advancement are coming that will allow players to
improve their characters and their gear in terms of resistances. That'd
all be pointless with these buffs still providing almost total immunity
so easily.

2 - The resistance values granted by these spells and rings has always
been rather absurd. A single resist buff lets you ignore an element.
A resist buff and a ring can make you completely immune.
Why not just cast all the buffs all the time and switch rings during combat?
Spell attacks are worse than useless in too many situations. I'm sure
Fike was not happy that people could sit in Nerudite Armor and resist buffs
and completely nullify his entire character without doing a thing.
Explosive Frosts should never be doing 1 damage. That's ridiculous.

So, with those in mind, I've tinkered with the resistance values
granted, and a few other things.

* Resist rings now grant 20% vs an element rather than 50%.

* All resist buffs are now self-cast only. Potions still work. They have
their own stomach/eating costs, so they're fine.
There are no potions for resist holy/unholy.

* Resist Magic is now 5% to 25% resistance, based on spellpower, down
from 40% max.

* Resist Fire, Cold, Shock, and Acid are also now 5% to 25% resist,
based on spellpower. They are inherently stronger, though, in that they
are lower level, of different schools, and stack with rings and other
gear resistances. These spells went as high as 65% and 75% before.

* Holy and Unholy Resolve now grant between 9% and 20% resistance to
their uncommon elements. They are weaker than the other resist buffs,
but now only cost a Fairy Wing instead of a Fairy Wing and Solagh. Most
of all, they can no longer be Purged. They are both strong enough to
exactly cover the holy/unholy weakness of ivies and orc shields. (20%)

These spells will be far more interesting and difficult to use now, for sure.
You can't simply cast every resist buff and go out unafraid of spell attacks.
For players truly wishing to gear up to fight mages, I recommend
Nerudite Armor + resist buffs + resist ring for 60% resistance to one element
and 40% to the rest. And 40%, if you notice, is what Resist Magic used to
reach at max power. So neru + resist buff replaces that efficacy.

For comparison, chain (25% pierce resist) + orc shield (15%) can reach
40% pierce resistance.

However, Nerudite Armor is -20% weak to Quake... so Faren mages will
have alternatives. A qor/faren can't purge you, sure, but he can certainly
get close and hit you for some serious damage with Earthquake if you let him.

As for holy and unholy resistance, these buffs can no longer be purged mainly
because it was a guaranteed damage type for Shals. Holy enchant a bow,
purge the holy resist buff + purge resist magic, and attack away - there are no
other sources of holy resistance in the game.
This simply wasn't fair, especially since ivy circlets previously granted 20% unholy
resistance, adding insult to injury for Qors trying to use unholy damage.
I've made ivy's unholy resist into a -20% unholy weakness to match orc shields'
-20% holy weakness. Both items are extremely powerful, and worth having a few drawbacks.