Rails 5 - Fix NotificationMailer Specs

Update NotificationMailer#find_categories_for specs to allow for Rails 5 typecasting of enum fields.
- Rails 5 introduces a change to ActiveRecord:Enum where it typecasts enum attributes to its string value. See: https://github.com/rails/rails/commit/c51f9b61ce1e167f5f58f07441adcfa117694301
- Luckily we can still send int values when doing a where clause on categories AND by Rails 5, we can also do a string array search of categories. So we do not need to update find_notifications_for
  - Decision Choice: another option instead of version checks is to typecast the categories from find_categories_for back to its corresponding ints (eg. .pluck('categories as i')). IMO, this makes the pluck a bit harder to understand and as mentioned in previous bullet point, receiving categories as strings does not seem to have a breaking domino effect. (because .where can accept either integer search or string search of enum fields).
- Regardless, we will test thoroughly in Staging Canary When Up.
Removed duplicate mailer method that uses .send in a way that is which is no longer valid in Ruby 2.6+
Add template_name parameter to mail to to specify template used. Due to a change in ActionMailer 4.2 -> Actionmailer 5.0

[x] Ensure NotificationMailer sends the correct template. (NotificationMailer used in NotificationEmailWorker which is enqueued via sidekiq-cron)

zooniverse / talk-api