FEATURE: Post deployment migrations. (#6406)

This moves us away from the delayed drops pattern which
was problematic on two counts. First, it uses a hardcoded "delay for"
duration which may be too short for certain deployment strategies.
Second, delayed drop doesn't ensure that it only runs after
the latest application code has been deployed. If the migration runs
and the application code fails to deploy, running the migration after
"delay for" has been met will cause the application to blow up.

The new strategy allows post deployment migrations to be skipped if the
env `SKIP_POST_DEPLOYMENT_MIGRATIONS` is provided.

```
SKIP_POST_DEPLOYMENT_MIGRATIONS=1 rake db:migrate
-> deploy app servers
SKIP_POST_DEPLOYMENT_MIGRATIONS=0 rake db:migrate
```

To aid with the generation of a post deployment migration, a generator
has been added. Simply run `rails generate post_migration`.

lib/discourse.rb

@@ -13,6 +13,7 @@ if Rails.env.development?
 end
 
 module Discourse
+  DB_POST_MIGRATE_PATH ||= "db/post_migrate"
 
   require 'sidekiq/exception_handler'
   class SidekiqExceptionHandler