Fix DB schema migrations

[apiraino]

• Fix a wrong migration introduced in Add webhook handler to update PR workload queues #1781
• Clarify how DB migrations should be added to the triagebot

r? @jackh726

[apiraino]

I can't check the schema of the triagebot in prod but the clauses IF NOT EXISTS should ensure nothing bad happens when applied.

[jackh726]

I think we should get eyes on this from @Mark-Simulacrum as I'm not very familiar with how migrations work and what the current state of the db is.

[ehuss]

Will this fix the errors about function sort(integer[]) does not exist?

[apiraino]

Will this fix the errors about function sort(integer[]) does not exist?

@ehuss Yes and now I know why. Without this patch the migration would fail:

$ cargo run --bin triagebot
...
Failed to run server: database migrations

Caused by:
    0: executing 12th migration
    1: db error: ERROR: cannot insert multiple commands into a prepared statement
    2: ERROR: cannot insert multiple commands into a prepared statement

unsure why it didn't happen (or wasn't catched) during the triagebot deployment at the time 🤷‍♂️

(I probably missed that on my local workstation because I manually fiddled with the DB schema without proberly testing the migration process)

EDIT: and that error we see in triagebot logs is triggered when from Zulip I request my PR work queue

[jackh726]

After talking with @Kobzol seems like this is fine