This reverts commit 13e37a3e.

Migrations shouldn't fail RuboCop checks - especially lint checks, such
as the nested method check. To avoid changing code in existing
migrations, add the magic comment to the top of each of them to skip
that file.

This reverts commit 3e991230.

# Conflicts:
#	app/models/project.rb

Since we eager load everything in lib/ putting ERB code in .rb files
will result in syntax errors. By moving the templates to
./generator_templates we can work around this.

This allows us to automatically include
Gitlab::Database::MigrationHelpers into newly generated migrations.