Hackathon: Post Reminders (#20555)

This PR adds the post reminder backend work.

We add a new API endpoint via which a user can set a reminder for a post. An ephemeral message will be sent down the line to let the user know about the action. And then after the time is over, the system admin bot will send a DM message to the user about the reminder post.

db/migrations/postgres/000091_create_post_reminder.down.sql

@@ -0,0 +1,3 @@
+DROP INDEX IF EXISTS idx_postreminders_targettime;
+
+DROP TABLE IF EXISTS postreminders;

db/migrations/postgres/000091_create_post_reminder.up.sql

@@ -0,0 +1,8 @@
+CREATE TABLE IF NOT EXISTS postreminders (
+    postid varchar(26) NOT NULL,
+    userid varchar(26) NOT NULL,
+    targettime bigint,
+    PRIMARY KEY (postid, userid)
+);
+
+CREATE INDEX IF NOT EXISTS idx_postreminders_targettime ON postreminders(targettime);