For sections, title is not required, but it's used to generate system_name, which is required. That makes title required in practice, since the UI doesn't provide a way to directly set the system_name. This PR adds the validation to make the title presence required.

This includes a migration to backfill all empty titles in the cms_sections table in the DB. The migration actually only fixes one record in production:

 
irb(main):001:0> CMS::Section.unscoped.where(title: ['', nil]).count
=> 1
 

A Release note is suggested:

"Developer portal: Sections without title are not allowed anymore. All untitled sections will be updated automatically and get a copy of the system name as title"

Check the PR to know how to test this: https://github.com/3scale/porta/pull/3140