models: fix deprecated errors[:base] << "message" calls.

Replace the idiom `errors[:base] << "message"` with
`errors.add(:base, "message")`. The former is deprecated in Rails 6.1.

app/controllers/emails_controller.rb

@@ -17,7 +17,7 @@ class EmailsController < ApplicationController
     if @user.authenticate_password(params[:user][:password])
       @user.update(email_address_attributes: { address: params[:user][:email] })
     else
-      @user.errors[:base] << "Password was incorrect"
+      @user.errors.add(:base, "Password was incorrect")
     end
 
     if @user.errors.none?

app/controllers/passwords_controller.rb

@@ -12,7 +12,7 @@ class PasswordsController < ApplicationController
     if @user.authenticate_password(params[:user][:old_password]) || @user.authenticate_login_key(params[:user][:signed_user_id])
       @user.update(password: params[:user][:password], password_confirmation: params[:user][:password_confirmation])
     else
-      @user.errors[:base] << "Incorrect password"
+      @user.errors.add(:base, "Incorrect password")
     end
 
     flash[:notice] = @user.errors.none? ? "Password updated" : @user.errors.full_messages.join("; ")

app/controllers/users_controller.rb

@@ -80,7 +80,7 @@ class UsersController < ApplicationController
       flash[:notice] = "Sign up failed"
     elsif @user.email_address&.invalid?(:deliverable)
       flash[:notice] = "Sign up failed: email address is invalid or doesn't exist"
-      @user.errors[:base] << @user.email_address.errors.full_messages.join("; ")
+      @user.errors.add(:base, @user.email_address.errors.full_messages.join("; "))
     elsif !@user.save
       flash[:notice] = "Sign up failed: #{@user.errors.full_messages.join("; ")}"
     else