Draft: Resolve "squash migrations script"

What does this MR do and why?

In this MR, we are introducing an automated way to squash old migrations.

How to run the script:

1. cd gitlab
2. scripts/squash-migrations MIGRATION_VERSION -> This will squash all migrations where <= MIGRATION_VERSION.

Script steps:

1. bundle exec rake db:drop db:create db:migrate VERSION=20190909141517 -> Drops the database; Creates a new database; Migrates to a specific version
2. bundle exec rake db:structure:dump SCHEMA=db/init_structure.sql -> Creates a new structure file with the name init_structure.sql
3. rake remove_old_migrations[20190909141517] -> Removes all migration files + specs (*)
4. bundle exec rake db:drop db:create db:migrate -> Creates a new database without using the old migrations

(*) We are not removing the init schema migration.

Note:

This only works if we change the content of the InitSchema class to something like this:

class InitSchema < ActiveRecord::Migration[6.0]
  def up 
    execute(IO.read("db/init_structure.sql"))
  end

  def down
    raise ActiveRecord::IrreversibleMigration, "The initial migration is not revertable"
  end
end

Questions:

1. Is it OK to always keep the /db/migrate/20181228175414_init_schema.rb file? I guess the most common strategy is to rename the last file that we want to squash to init schema.
2. execute(IO.read("db/init_structure.sql")) -> I am not sure if this creates a security issue, since we are reading sql instructions from a file.
3. I am not sure if we can use the plain db/init_structure.sql file. I need to double-check this.

What do you think @abrandl @pbair?

Btw: This is just a draft. I would like to hear your opinion first before I start to refactor and add tests to the code :)

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Related to #331248 (closed)