mark grouped notifs by oldest id - sort-of fix 1139

Misskey's code does the same, but our groups behave differently enough
that this may be not the best choice

for example, let's say we have:

- notifications 1-5 for reaction to note A
- notifications 6-8 for reaction to note B
- notifications 9-12 for reaction to note A
- notification 13-19 for non-groupable events
- notification 20 for reaction to note A

and that events happened one every minute (so the last notification is
from 20 minutes ago)

client requests the most recent 10 notifications; we fetch
notifications 1-10, and reply:

- grouped id 6 for reactions 6-8 to note B
- grouped id 10 for reactions 1-5, 9-10 to note A

then the client requests 10 more notifications, untilId=10; we fetch
notifications 11-20, and reply:

- non-grouped notifications 13-19
- grouped id 20 for reactions 11,12,20 to note A

because we sort by id, and also the `createdAt` marks the _newest_
event in each group, the client will then show:

  6 reactions to note B, 6 minutes ago
  4 reactions to note A, 1 minute ago
  notifications 13-19, 13 minutes to 19 minutes ago
  3 reactions to note A, 11 minutes ago

I don't know how to make this work better ☹

0 files changed, 0 insertions, 0 deletions