add create and destroy services for emails

app/services/emails/base_service.rb

+module Emails
+  class BaseService
+    def initialize(current_user, user, opts)
+      @current_user = current_user
+      @user = user
+      @email = opts[:email]
+    end
+
+    private
+
+    def can_manage_emails?
+      @current_user == @user || @current_user.admin?
+    end
+  end
+end

app/services/emails/create_service.rb

+module Emails
+  class CreateService < BaseService
+    def execute(skip_authorization: false)
+      raise Gitlab::Access::AccessDeniedError unless skip_authorization || can_manage_emails?
+
+      @user.emails.create!(email: @email)
+    end
+  end
+end

app/services/emails/destroy_service.rb

+module Emails
+  class DestroyService < BaseService
+    def execute(skip_authorization: false)
+      raise Gitlab::Access::AccessDeniedError unless skip_authorization || can_manage_emails?
+
+      Email.find_by_email(@email).destroy!
+    end
+  end
+end