Move lib folder inside the app folder

The purpose of the lib folder is to have code that doesn't necessary
belong in the application but can be shared with other applications.

However, we don't have other applications and, if we did, the way to
share code between them would be using a gem or even a git submodule.

So having both the `app/` and the `lib/` folders is confusing IMHO, and
it causes unnecessary problems with autoloading.

So we're moving the `lib/` folder to `app/lib/`. Originally, some of
these files were in the `app/services/` folder and then they were moved
to the `lib/` folder. We're using `app/lib/` instead of `app/services/`
so the upgrade is less confusing.

There's an exception, though. The `OmniAuth::Strategies::Wordpress`
class needs to be available in the Devise initializer. Since this is an
initializer and trying to autoload a class here will be problematic when
switching to Zeitwerk, we'll keep the `require` clause on top of the
Devise initializer in order to load the file and so it will be loaded
even if it isn't in the autoload paths anymore.

app/lib/email_digest.rb

@@ -0,0 +1,31 @@
+class EmailDigest
+  attr_accessor :user
+
+  def initialize(user)
+    @user = user
+  end
+
+  def notifications
+    user.notifications.not_emailed.where(notifiable_type: "ProposalNotification")
+  end
+
+  def pending_notifications?
+    notifications.any?
+  end
+
+  def deliver(run_at)
+    if valid_email? && pending_notifications?
+      Mailer.delay(run_at: run_at).proposal_notification_digest(user, notifications.to_a)
+      mark_as_emailed
+    end
+  end
+
+  def mark_as_emailed
+    notifications.update_all(emailed_at: Time.current)
+    user.update!(failed_email_digests_count: 0)
+  end
+
+  def valid_email?
+    user.email.present? && user.email.match(/\A([^@\s]+)@((?:[-a-z0-9]+\.)+[a-z]{2,})\Z/i)
+  end
+end