Draft: Speed-up the gitlab-triage-gardener script

refs sysadmin#17974 (closed)

Problem: the current way of using gitlab-trage "summary policies" to join per-user sub-policies does one query per user per policy, which currently amounts to ~1.5K queries and takes ages to complete.

We can instead avoid using "summary policies" and look at relevant issues first, bringing the amount of queries to 3 (modulo pagination, which should be very infrequent).

Side effects:

• Issues are not grouped by user anymore.
• Unassigned issues and MRs are now also reported in the summary issues now.

Other changes:

• Make text of summaries more concise.
• Use comments/threads instead of inline lists of issues.

To do:

• Double check that the shortened versions keep the spirit of the previous texts
• Implement suggestions discussed with @segfault