DEV: Add annotate rake tasks, and enforce via GitHub actions

`bin/rake annotate` is an alias of `bin/annotate --models`
`bin/rake annotate:clean` generates annotations by using a temporary, freshly migrated database. This should help us to produce more consistent annotations, even if development databases have been polluted by plugin migrations.

A GitHub actions task is also added which generates annotations on a clean database, and raises an error if they differ from the committed annotations.

Gemfile

@@ -165,6 +165,8 @@ group :test, :development do
   gem 'parallel_tests'
 
   gem 'rswag-specs'
+
+  gem 'annotate'
 end
 
 group :development do
@@ -173,7 +175,6 @@ group :development do
   gem 'better_errors', platform: :mri, require: !!ENV['BETTER_ERRORS']
   gem 'binding_of_caller'
   gem 'yaml-lint'
-  gem 'annotate'
   gem 'discourse_dev_assets'
   gem 'faker', "~> 2.16"
 end