Auto archiving channels on closed issues

[fsergot]

This would allow to not pile closed channels on Slack.
A first draft was made but was a little complex. The logic was:

• Allow admin to setup a 'scope' JQL for archiving (typically all issue with slack channel & resolved for more than 3 month)
• Add a jira job running on a regular basis
• From this job, execute the SQL and for each issue, check the associated slack channel and archive it

This requires additional setup also for a slack api permanent token (as the jira job runs as background, no interactive user)
This would also require evolution of the slack field to store the fact that the related channel is archived (to avoid re-runs and also adapt the view section on jira)