Send reminders

For this final reminders feature, I setup Sidekiq to run background processes for selected reminder options for appointments. Users are able to select none or all reminder options to be reminded of their appointments at its specified time. The following reminder options are: Email, Phone Number, and Text messages. For email, I utilize the Sendgrid starter addon to send emails to users. For phone calls and text messages, I use the Twilio trial version. Unfortunately, at this stage, only phone numbers that are white-listed from Twilio can be contacted due to my trial version.
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