DEV: Set limits on custom fields

This patch sets some limits on custom fields:
- an entity can’t have more than 100 custom fields defined on it
- a custom field can’t hold a value greater than 10,000,000 characters

The current implementation of custom fields is relatively complex and
does an upsert in SQL at some point, thus preventing to simply add an
`ActiveRecord` validation on the custom field model without having to
rewrite a part of the existing logic.
That’s one of the reasons this patch is implementing validations in the
`HasCustomField` module adding them to the model including the module.

spec/models/user_spec.rb

@@ -1,9 +1,11 @@
 # frozen_string_literal: true
 
 RSpec.describe User do
+  subject(:user) { Fabricate(:user, last_seen_at: 1.day.ago) }
+
   fab!(:group) { Fabricate(:group) }
 
-  subject(:user) { Fabricate(:user, last_seen_at: 1.day.ago) }
+  it_behaves_like "it has custom fields"
 
   def user_error_message(*keys)
     I18n.t(:"activerecord.errors.models.user.attributes.#{keys.join(".")}")