issues.title and merge_requests.title are missing a length constraint

The module Issuable adds a validation on :title to ensure the length is not greater than 255:

validates :title, presence: true, length: { maximum: 255 }

This constraint however is not present on database level, or at least not in db/schema.rb. We should add this constraint to both issues.title and merge_requests.title so there's no way of ever inserting data that's longer than 255 characters.