Create table user_credit_card_validations

What does this MR do?

Related to issue: https://gitlab.com/gitlab-org/gitlab/-/issues/329141

This MR: create a new table user_credit_card_validations, and add/update model files(User, UserCreditCardValidation) accordingly.

Why create a new table?

We need to save a new timestamp information (credit_card_validated_at) for user. This information will be used to reduce the abuse of CI pipeline usage. We choose to create a new table user_credit_card_validations, instead of adding a new column credit_card_validated_at directly to users table. Because:

• We have >8M users today in production DB. Only small number of users will need to verify their credit card, which means for most users, their credit_card_validated_at will be null.
• We try to avoid adding new columns to users, because it already has too many columns.

Migration output

$ bin/rails db:migrate
== 20210429131525 CreateUserCreditCardValidations: migrating ==================
-- create_table(:user_credit_card_validations, {:id=>false})
   -> 0.0201s
== 20210429131525 CreateUserCreditCardValidations: migrated (0.0253s) =========


$ bin/rails db:rollback
== 20210429131525 CreateUserCreditCardValidations: reverting ==================
-- drop_table(:user_credit_card_validations)
   -> 0.0026s
== 20210429131525 CreateUserCreditCardValidations: reverted (0.0101s) =========

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