Fix RSS feed tokens for issues list

What does this MR do and why?

This MR adds the feed_token param to the group / project RSS endpoints. These were included in the legacy lists, but were missed in the transition to the new work item powered lists.

References

• !205308 (comment 2780078494)

Screenshots or screen recordings

Not provided

How to set up and validate locally

1. Enable the work_items_group_issues_list feature flag
2. As a logged in user, navigate to the project issues list
3. In the ⋮ menu on the top right, select Subscribe to RSS feed
4. Observe the URL contains the feed token (e.g: ?feed_token=glft-xxxx)
5. Repeat the process above for the group issues list. Observe the same behaviour

Note that the feed tokens should match those in the legacy lists. You can verify this by disabling both the work_items_group_issues_list and work_items_project_issues_list feature flags, repeating the steps above, and comparing the feed token values.

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