Resolve "[Rails5] Make GeoMigrationGenerator compatible with Rails 5"

What does this MR do?

This MR changes the location of the generator templates that will be used from within activerecord gem to the internal generator templates path.

Example of new Geo DB migration:

$ bundle exec rails g geo_migration bleep1
DEPRECATION WARNING: alias_method_chain is deprecated. Please, use Module#prepend instead. From module, you can access the original method using super. (called from <main> at /Users/ash/src/gdk/gdk-geo/gitlab/config/application.rb:5)
      create  ee/db/geo/migrate/20180904034333_bleep1.rb

ee/db/geo/migrate/20180904034333_bleep1.rb:

# frozen_string_literal: true

# See http://doc.gitlab.com/ce/development/migration_style_guide.html
# for more information on how to write migrations for GitLab.

class Bleep1 < ActiveRecord::Migration
  include Gitlab::Database::MigrationHelpers

  # Set this constant to true if this migration requires downtime.
  DOWNTIME = false

  # When a migration requires downtime you **must** uncomment the following
  # constant and define a short and easy to understand explanation as to why the
  # migration requires downtime.
  # DOWNTIME_REASON = ''

  # When using the methods "add_concurrent_index", "remove_concurrent_index" or
  # "add_column_with_default" you must disable the use of transactions
  # as these methods can not run in an existing transaction.
  # When using "add_concurrent_index" or "remove_concurrent_index" methods make sure
  # that either of them is the _only_ method called in the migration,
  # any other changes should go in a separate migration.
  # This ensures that upon failure _only_ the index creation or removing fails
  # and can be retried or reverted easily.
  #
  # To disable transactions uncomment the following line and remove these
  # comments:
  # disable_ddl_transaction!

  def change
  end
end

What are the relevant issue numbers?

#7074 (closed)

Does this MR meet the acceptance criteria?

• [ ] Changelog entry added, if necessary
• [ ] Documentation created/updated
• [ ] Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• EE specific content should be in the top level /ee folder
• [ ] For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?

Closes #7074 (closed)