INTL-323 Site-specific migrations

CI: N/A, repo has no build script
QA:

This PR introduces a mechanism to ensure that all data migrations specify which site they are targeted at (UK, DE or both). This is to reduce the risk of wrongly running a migration against the wrong site.

This only applies to data migrations, not schema migrations.

To specify which site a migration is targeting, use `for_site`:
```rb
class MyMigration < ActiveRecord::Migration
  for_site :uk, :de
  def up
     ...
  end
end
```
If you try to run a data migration which doesn't specify a site, it will raise an `InvalidSiteMigration` error.

To allow developers to run older data migrations, which do not specify a site, there is a `SKIP_MIGRATION_SITE_CHECK` environment variable to override the check. 

With credit to @ohthatjames for making sense of Rails' migration internals.

[Previous PR in noths-rails-extensions with discussion](https://github.com/notonthehighstreet/noths-rails-extensions/pull/28)