No fully qualified identifiers, please, IntelliJ >_>
Rewrite and rename Collection -> String method.
Implement Grantable entry fees [BREAKING]

// BREAKING
This commit changes the return type of the getEntryFee() method in the
Arena interface (and ArenaImpl class).
// BREAKING

This commit converts the entry fee system to the new Grantable system.
As a result, it is now possible to use arbitrary permissions as a type
of "token" to enter an arena. The "tokens" could possibly be granted
as rewards for reaching certain waves in always-accessible arenas. It
is also possible to use potion effects as entry fees, which is maybe a
bit strange, but possible, nonetheless.
Tweak grantable Effect parsing.

Parsing an effect with amplifier/level 1 now correctly creates a level
1 effect instead of level 2. Thus, the parser now accepts 1-indexed
amplifiers/levels instead of 0-indexed ones.

Durations lower than or equal to 0 are parsed as Integer.MAX_VALUE for
easy "infinite" effects.
Add support for potion effects on classes.