@YuukanOO, try with v1.2-RC1 or wait until v1.2 releases next Monday and the Slack UI should work, there was a bug in v1.1 on the specific scenario you're describing.
@jbick, if you decide to open source your Jira integration, please let us know, we'd love to share your work with the community via www.mattermost.org/community-applications/