[grouping info] Include Seer as grouping reason

When Seer is used to assign an event to an existing group, we give no indication of that in the grouping info, but we should.

UPDATE: We do now show it in the merged issues tab, which is a good start, but leaving this open to track actually adding it to grouping info.

Start by tracing how grouping information is generated when Seer assigns an event to an existing group, then compare that path with the merged issues tab where Seer is already shown. Done means grouping info explicitly indicates Seer as the grouping reason for those events.

Written by the indexing model from the issue text.