dmails, emails: refactor to use Rails signed_id.

Refactor email verification links and Dmail share links to use the new
Rails signed_id mechanism, rather than our own handrolled mechanism.

For Dmail share links, we have to override some Rails internal methods
so that our old links still work. For email verification links, this
will invalidate existing links, but this isn't a huge deal since these
links are short-lived anyway.

https://api.rubyonrails.org/classes/ActiveRecord/SignedId.html
https://api.rubyonrails.org/classes/ActiveRecord/SignedId/ClassMethods.html

app/controllers/application_controller.rb

@@ -99,6 +99,8 @@ class ApplicationController < ActionController::Base
       render_error_page(401, exception, template: "sessions/new")
     when ActionController::InvalidAuthenticityToken, ActionController::UnpermittedParameters, ActionController::InvalidCrossOriginRequest
       render_error_page(403, exception)
+    when ActiveSupport::MessageVerifier::InvalidSignature # raised by `find_signed!`
+      render_error_page(403, exception, template: "static/access_denied", message: "Access denied")
     when User::PrivilegeError, Pundit::NotAuthorizedError
       render_error_page(403, exception, template: "static/access_denied", message: "Access denied")
     when ActiveRecord::RecordNotFound

app/controllers/dmails_controller.rb

@@ -20,7 +20,11 @@ class DmailsController < ApplicationController
   end
 
   def show
-    @dmail = authorize Dmail.find(params[:id])
+    if params[:key].present?
+      @dmail = Dmail.find_signed!(params[:key], purpose: "dmail_link")
+    else
+      @dmail = authorize Dmail.find(params[:id])
+    end
 
     if request.format.html? && @dmail.owner == CurrentUser.user
       @dmail.update!(is_read: true)

app/controllers/emails_controller.rb

@@ -48,8 +48,7 @@ class EmailsController < ApplicationController
 
     if @email_address.blank?
       redirect_to edit_user_email_path(@user)
-    elsif params[:email_verification_key].present?
-      authorize @email_address
+    elsif params[:email_verification_key].present? && @email_address == EmailAddress.find_signed!(params[:email_verification_key], purpose: "verify")
       @email_address.verify!
       flash[:notice] = "Email address verified"
       redirect_to @email_address.user