Task lists in Milestone description are disabled...

Notes

• Task lists / checkboxes seem to be disabled in Milestone descriptions
• Also noted that the model Milestone does not have a lock_version that avoids conflicts when updating the milstone object

Proposal / Possible Implementation

• Introduce a field lock_version
• We need to use the js component Tasklist, see https://gitlab.com/gitlab-community/gitlab/blob/58f7ae53d4f0fb52c2b86bfd94b70ada9e1c36ca/app/assets/javascripts/task_list.js#L37

Checklist

• Enable task list, see Enable task list in milestone show view (!118927 - merged)
• Add field lock_version to model Milestone, see Add lock_version to milestone (!119859 - merged)
• Add tests