Add plan_name_uid column to plans table

What does this MR do and why?

This MR adds plan_name_uid to plans table. This column will be used to convert the plans table to have hard-coded primary key and globally unique reference for Cells infrastructure.

• Plans table is a small table with total 12 rows only.
• new records can only inserted via database migration
  • Example MR: !135759 (diffs)
• Thus, there is no significant application code that creates new record, or updates existing record.
• Only two places seems to have find_or_create for default plan and free plan
  • Plan.default
    • The default plan is already present in Gitlab.com - SaaS
    • And for self-managed, it is seeded during initialization
  • Plan.free
    • This is only for SaaS - Gitlab.com and we already have this record in the database created
  • In any case, we have before_validate hook to set the plan if needed for development scripts.

Database

gitlabhq_dblab=# select name, title from plans;
             name             |              title
------------------------------+----------------------------------
 default                      | Default
 early_adopter                | GitLab.com Early Adopter
 bronze                       | Bronze
 silver                       | Silver
 gold                         | Gold
 free                         | Free
 ultimate_trial               | Ultimate Trial
 premium_trial                | Premium Trial
 opensource                   | Open Source Program
 premium                      | Premium
 ultimate                     | Ultimate
 ultimate_trial_paid_customer | Ultimate Trial for Paid Customer
(12 rows)

References

• Add name_uid enum column to plans table with ba... (#571419 - closed)

How to set up and validate locally

• Green CI

MR acceptance checklist

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