Draft: Add reminders table

What does this MR do and why?

This table will back a new Reminder class that will store scheduled Reminder events, to be processed by an as-yet unwritten worker. This is a piece in supporting &13628

Database

Up

main: == 20240426181620 CreateReminders: migrating ==================================
main: -- create_table(:reminders)
main: -- quote_column_name(:target_type)
main:    -> 0.0000s
main: -- quote_column_name(:message)
main:    -> 0.0000s
main:    -> 0.0100s
main: == 20240426181620 CreateReminders: migrated (0.0299s) =========================
main: == 20240426194611 AddFkToRemindersTable: migrating ============================
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- execute("ALTER TABLE reminders ADD CONSTRAINT fk_49f81d5e52 FOREIGN KEY (user_id) REFERENCES users (id) ON DELETE CASCADE NOT VALID;")
main:    -> 0.0018s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0002s
main: -- execute("ALTER TABLE reminders VALIDATE CONSTRAINT fk_49f81d5e52;")
main:    -> 0.0022s
main: -- execute("RESET statement_timeout")
main:    -> 0.0005s
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- execute("ALTER TABLE reminders ADD CONSTRAINT fk_d32035765e FOREIGN KEY (project_id) REFERENCES projects (id) ON DELETE CASCADE NOT VALID;")
main:    -> 0.0010s
main: -- execute("ALTER TABLE reminders VALIDATE CONSTRAINT fk_d32035765e;")
main:    -> 0.0029s
main: == 20240426194611 AddFkToRemindersTable: migrated (0.0466s) ===================

Down

main: == 20240426194611 AddFkToRemindersTable: reverting ============================
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- remove_foreign_key(:reminders, {:column=>:user_id})
main:    -> 0.0033s
main: -- remove_foreign_key(:reminders, {:column=>:project_id})
main:    -> 0.0022s
main: == 20240426194611 AddFkToRemindersTable: reverted (0.0488s) ===================
main: == 20240426181620 CreateReminders: reverting ==================================
main: -- drop_table(:reminders)
main:    -> 0.0023s
main: == 20240426181620 CreateReminders: reverted (0.0076s) =========================

Related to #457815 (closed)