Validate label's title length

What does this MR do?

Adds validation of label.title.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Error is fired in case user tries to add label with very long title (> 255 chars)

ActiveRecord::StatementInvalid (Mysql2::Error: Data too long for column 'title' at row 1: INSERT INTO `labels` (`title`, `description`, `color`, `project_id`, `created_at`, `updated_at`) VALUES ('Or you can choose one of suggested colors below Or you can choose one of suggested colors below Or you can choose one of suggested colors below Or you can choose one of suggested colors below Or you can choose one of suggested colors below Or you can choose one of suggested colors below Or you can choose one of suggested colors below Or you can choose one of suggested colors below Or you can choose one of suggested colors below Or you can choose one of suggested colors below Or you can choose one of suggested colors below Or you can choose one of suggested colors below Or you can choose one of suggested colors below Or you can choose one of suggested colors below Or you can choose one of suggested colors below Or you can choose one of suggested colors below Or you can choose one of suggested colors below Or you can choose one of suggested colors below Or you can choose one of suggested colors below Or you can choose one of suggested colors below Or you can choose one of suggested colors below Or you can choose one of suggested colors below Or you can choose one of suggested colors below ', '', '#428bca', 128, '2016-08-11 07:40:56', '2016-08-11 07:40:56')):
  config/initializers/connection_fix.rb:20:in `execute'
  app/controllers/projects/labels_controller.rb:30:in `create'
  lib/gitlab/middleware/go.rb:16:in `call

What are the relevant issue numbers?

#20809 (closed)

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together