devise.rb Initialzer not generated / not configuring mailer correctly

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Followed the setup tutorial https://github.com/dnasseri/rails5-api-guide.

However, when setting up mailer config, I am unable to locate the devise.rb initializer to add the following information:

```
Devise.setup do |config|
# Using rails-api, tell devise to not use ActionDispatch::Flash
# middleware b/c rails-api does not include it.
config.navigational_formats = [:json]
end
```

I created a new devise.rb file with the above content. However, when running the following curl command:

```
curl --data "email=test3@test.com&password=testpassword1&password_confirmation=testpassword1&confirm_success_url=http://localhost:3000" http://localhost:3000/auth
```

I end up with the following in my logs:

> Started POST "/auth" for 127.0.0.1 at 2018-04-10 23:35:13 +0200
> Processing by DeviseTokenAuth::RegistrationsController#create as */*
> Parameters: {"email"=>"test3@test.com", "password"=>"[FILTERED]", "password_confirmation"=>"[FILTERED]", "confirm_success_url"=>"http://localhost:3000"}
>  (0.2ms) BEGIN
> User Exists (0.5ms) SELECT 1 AS one FROM "users" WHERE "users"."email" = $1 AND "users"."provider" = $2 LIMIT $3 [["email", "test3@test.com"], ["provider", "email"], ["LIMIT", 1]]
> SQL (0.5ms) INSERT INTO "users" ("uid", "encrypted_password", "email", "tokens", "created_at", "updated_at") VALUES ($1, $2, $3, $4, $5, $6) RETURNING "id" [["uid", "test3@test.com"], ["encrypted_password", "$2a$11$//lH35X2BeoWxoYSlzQFLul36L8SU0DVecPglxH6GoentGqM/BA3y"], ["email", "test3@test.com"], ["tokens", "{}"], ["created_at", "2018-04-10 21:35:13.828569"], ["updated_at", "2018-04-10 21:35:13.828569"]]
>  (19.9ms) COMMIT
>  (0.3ms) BEGIN
> SQL (0.6ms) UPDATE "users" SET "tokens" = $1, "updated_at" = $2 WHERE "users"."id" = $3 [["tokens", "{\"zYT85oY4RmjKlNQODIzZoA\":{\"token\":\"$2a$10$6qPUooSqcVTjG6lQ/hiZ3e4rIrN6uPC44IE1DkbHODk47GYiwl4K2\",\"expiry\":1524605713}}"], ["updated_at", "2018-04-10 21:35:13.971799"], ["id", 3]]
>  (9.7ms) COMMIT
>  (0.3ms) BEGIN
> User Load (0.9ms) SELECT "users".* FROM "users" WHERE "users"."id" = $1 LIMIT $2 FOR UPDATE [["id", 3], ["LIMIT", 1]]
> SQL (0.6ms) UPDATE "users" SET "tokens" = $1, "updated_at" = $2 WHERE "users"."id" = $3 [["tokens", "{\"zYT85oY4RmjKlNQODIzZoA\":{\"token\":\"$2a$10$Hb6cME0d7HtACWnypdBD0OoT/VoroGMrB6dJrGdrjnfMgXcKEZHsy\",\"expiry\":1524605713,\"last_token\":\"$2a$10$6qPUooSqcVTjG6lQ/hiZ3e4rIrN6uPC44IE1DkbHODk47GYiwl4K2\",\"updated_at\":\"2018-04-10T23:35:13.987+02:00\"}}"], ["updated_at", "2018-04-10 21:35:14.107194"], ["id", 3]]
>  (6.8ms) COMMIT
> Completed 200 OK in 535ms (Views: 0.4ms | ActiveRecord: 40.3ms)

However, I expect mailer information as outlined in the setup tutorial:

> Started POST "/auth" for ::1 at 2016-07-23 19:16:09 -0400
> Processing by DeviseTokenAuth::RegistrationsController#create as */*
> Parameters: {"email"=>"test@test.com", "password"=>"[FILTERED]", "password_confirmation"=>"[FILTERED]", "confirm_success_url"=>"http://localhost:3000"}
> -- snip --
> Devise::Mailer#confirmation_instructions: processed outbound mail in 170.2ms
> Sent mail to test@test.com (8.4ms)
> Date: Sat, 23 Jul 2016 19:16:10 -0400
> From: please-change-me-at-config-initializers-devise@example.com
> Reply-To: please-change-me-at-config-initializers-devise@example.com
> To: test@test.com
> Message-ID:
> Subject: Confirmation instructions
> Mime-Version: 1.0
> Content-Type: text/html;
> charset=UTF-8
> Content-Transfer-Encoding: 7bit
> client-config: default
> redirect-url: http://localhost:3000
>
>

Welcome test@test.com!

>
>

You can confirm your account email through the link below:

>
>

Confirm my account

>
> Completed 200 OK in 666ms (Views: 0.6ms | ActiveRecord: 35.4ms)

Start with the setup tutorial linked in the issue and reproduce the registration request using the provided curl command. Compare the observed registration logs with the expected Devise::Mailer confirmation output; done means the configured registration flow produces the expected mailer activity and confirmation details.

Written by the indexing model from the issue text.