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/migration/safe_migrate.rb

@@ -16,13 +16,33 @@ class Migration::SafeMigrate
     end
 
     def migrate(direction)
-      if direction == :up && version && version > UNSAFE_VERSION && @@enable_safe != false
+      if direction == :up &&
+         version && version > UNSAFE_VERSION &&
+         @@enable_safe != false &&
+         !is_post_deploy_migration?
+
         Migration::SafeMigrate.enable!
       end
+
       super
     ensure
       Migration::SafeMigrate.disable!
     end
+
+    private
+
+    def is_post_deploy_migration?
+      method =
+        if self.respond_to?(:up)
+          :up
+        elsif self.respond_to?(:change)
+          :change
+        end
+
+      self.method(method).source_location.first.include?(
+        Discourse::DB_POST_MIGRATE_PATH
+      )
+    end
   end
 
   module NiceErrors
@@ -90,7 +110,7 @@ class Migration::SafeMigrate
         -------------------------------------------------------------------------------------
         An attempt was made to drop or rename a table in a migration
         SQL used was: '#{sql}'
-        Please use the deferred pattern using Migration::TableDropper in db/seeds to drop
+        Please generate a post deployment migration using `rails g post_migration` to drop
         or rename the table.
 
         This protection is in place to protect us against dropping tables that are currently
@@ -103,7 +123,8 @@ class Migration::SafeMigrate
         -------------------------------------------------------------------------------------
         An attempt was made to drop or rename a column in a migration
         SQL used was: '#{sql}'
-        Please use the deferred pattern using Migration::ColumnDropper in db/seeds to drop
+
+        Please generate a post deployment migration using `rails g post_migration` to drop
         or rename columns.
 
         Note, to minimize disruption use self.ignored_columns = ["column name"] on your