Migration: add trial extension type to gitlab_subscription

What does this MR do?

Related to issue #290278 (closed).

We need a flag to tell the GitlabSubscription trial extension type:

• nil: trial is not extended or reactivated
• 1: trial is an extended trial
• 2: trial is a reactivated trial

The same flag is added for Order in CustomersDot DB by https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/2937

Migrating up output:

$ bin/rails db:migrate
== 20210316094047 AddTrialExtensionTypeToGitlabSubscriptionHistories: migrating 
-- add_column(:gitlab_subscription_histories, :trial_extension_type, :smallint)
   -> 0.0022s
== 20210316094047 AddTrialExtensionTypeToGitlabSubscriptionHistories: migrated (0.0022s) 

== 20210316094323 AddTrialExtensionTypeToGitlabSubscriptions: migrating =======
-- add_column(:gitlab_subscriptions, :trial_extension_type, :smallint)
   -> 0.0013s
== 20210316094323 AddTrialExtensionTypeToGitlabSubscriptions: migrated (0.0060s) 

Rollback output:

$ bin/rails db:rollback
== 20210316094323 AddTrialExtensionTypeToGitlabSubscriptions: reverting =======
-- remove_column(:gitlab_subscriptions, :trial_extension_type)
   -> 0.0010s
== 20210316094323 AddTrialExtensionTypeToGitlabSubscriptions: reverted (0.0134s) 


$ bin/rails db:rollback
== 20210316094047 AddTrialExtensionTypeToGitlabSubscriptionHistories: reverting 
-- remove_column(:gitlab_subscription_histories, :trial_extension_type, :smallint)
   -> 0.0023s
== 20210316094047 AddTrialExtensionTypeToGitlabSubscriptionHistories: reverted (0.0036s) 

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