bans: validate ban duration is standard length.

app/models/ban.rb

@@ -12,6 +12,8 @@ class Ban < ApplicationRecord
   belongs_to :user
   belongs_to :banner, :class_name => "User"
 
+  validates :duration, presence: true
+  validates :duration, inclusion: { in: [1.day, 3.days, 1.week, 1.month, 3.months, 6.months, 1.year, 100.years], message: "%{value} is not a valid ban duration" }, if: :duration_changed?
   validates :reason, presence: true
   validate :user, :validate_user_is_bannable, on: :create