SLA is not enabled for closed issues

Summary of bug

When enabling Service Desk for an existing project, if there are already existing issues that are closed, those issues will not have the SLA counter, and as such will not be counted into the SLA reports

Steps to reproduce

1. Create a project and a few issues
2. Close some of the issues (but not all)
3. Enable Service Desk for that project
4. Check the closed issues, there will not be any SLA there.

An example :

The project PNS has 5 issues :

When searching for

project = PNS and "Time to resolution" = breached()

the results contains 1 issue :

When searching for

project = PNS and "Time to resolution" != breached()

the results contains 3 issues :

There is one more issue missing, and that issue was actually closed before Service Desk was enabled for this project. As such, the results from the two queries does not add up to the total amount of issues contained in the project.

Additional Info

• Although the closed issues will not have any SLA, the issues that are not closed yet will have SLA
• If the closed issues are re-opened, then SLA will be enabled for it. However, the SLA will be reset
• When Service Desk is activated for a project, the SLA information is actually already stored in the database, even for the closed issues. However, for some reason, it is not showing in the UI of the issue.