emails: add /emails index page.

Add emails index page at https://danbooru.donmai.us/emails. Mods can use
this page to view and search emails belonging to users below mod level.

app/models/email_address.rb

@@ -10,6 +10,14 @@ class EmailAddress < ApplicationRecord
   validate :validate_deliverable, on: :deliverable
   after_save :update_user
 
+  def self.visible(user)
+    if user.is_moderator?
+      where(user: User.where("level < ?", user.level)).or(where(user: user))
+    else
+      none
+    end
+  end
+
   def address=(value)
     self.normalized_address = EmailValidator.normalize(value) || address
     super
@@ -29,6 +37,15 @@ class EmailAddress < ApplicationRecord
     user.update!(is_verified: is_verified? && nondisposable?)
   end
 
+  def self.search(params)
+    q = super
+
+    q = q.search_attributes(params, :user, :address, :normalized_address, :is_verified, :is_deliverable)
+    q = q.apply_default_order(params)
+
+    q
+  end
+
   concerning :VerificationMethods do
     def verifier
       @verifier ||= Danbooru::MessageVerifier.new(:email_verification_key)