Update event push action and receipt tables to support threads. (#13753)

Adds a `thread_id` column to the `event_push_actions`, `event_push_actions_staging`,
and `event_push_summary` tables. This will notifications to be segmented by the thread
in a future pull request. The `thread_id` column stores the root event ID or the special
value `"main"`.

The `thread_id` column for `event_push_actions` and `event_push_summary` is
backfilled with `"main"` for all existing rows. New entries into `event_push_actions`
and `event_push_actions_staging` will get the proper thread ID.

`receipts_linearized` and `receipts_graph` also gain a `thread_id` column, which is similar,
except `NULL` is a special value meaning the receipt is "unthreaded".

See MSC3771 and MSC3773 for where this data will be useful.

1 files changed, 1 insertions, 0 deletions

diff --git a/changelog.d/13753.misc b/changelog.d/13753.misc
new file mode 100644
index 0000000000..63de2eb9f9
--- /dev/null
+++ b/changelog.d/13753.misc
@@ -0,0 +1 @@
+Prepatory work for storing thread IDs for notifications and receipts.