Require minimum password length of 4, and trim new passwords for whitespace.

Fixes problems reported by @MarioE and @Simon311:
- Users can no longer register with whitespace for a password.
- Users can no longer register with 4 whitespaces for a password.

TShockAPI/ConfigFile.cs

@@ -401,6 +401,9 @@ namespace TShockAPI
 		[Description("Determines the BCrypt work factor to use. If increased, all passwords will be upgraded to new work-factor on verify. Range: 5-31.")]
 		public int BCryptWorkFactor = 7;
 
+		[Description("The minimum password length for new user accounts. Minimum value is 4.")]
+		public int MinimumPasswordLength = 4;
+
 		/// <summary>
 		/// Reads a configuration file from a given path
 		/// </summary>