JIRA: second round

We have improved our support of JIRA in 8.13, we will continue in 8.14 with the following:

• Add remote issue links to JIRA https://gitlab.com/gitlab-org/gitlab-ce/issues/13232
• Configurable JIRA plugin https://gitlab.com/gitlab-org/gitlab-ce/issues/5541
• Change transition input field to a dropdown listing all the possible transition https://gitlab.com/gitlab-org/gitlab-ce/issues/23454
• API documentation: Update JIRA attributes available via API https://gitlab.com/gitlab-org/gitlab-ee/issues/440
• Bug: Issues can't be closed after JIRA integration https://gitlab.com/gitlab-org/gitlab-ce/issues/1981