Bulk merge improvement: allow selecting tickets without filtering by `status`:`open`

Created by: malomarrec

Current behaviour

Merge is only available if filtering by state open docs This makes the feature hard to discover.

Proposed behaviour

• Merge is always visible, but the button is either active or disabled depending on the selection
• Merge is available regardless of what status is selected
• If only open changesets are selected, the merge button is active
• If any changeset with a status different from open is selected, the merge button is disabled