codemonitors: Clean up CodeMonitorStore

Created by: camdencheek

Welcome to this monster housecleaning PR for CodeMonitor store. I started making small housecleaning PRs to bring our CodeMonitorStore up to current standards and make it a more idiomatic store, but there were many changes, so I decided it was better to rip off the band-aid and make one massive PR with all the changes. I could make these commits all separate PRs, but then it would take all week to get them reviewed, rebased, and merged.

A brief tour of what you'll find in this PR:

• Renaming methods to be more consistent and read more clearly (see Glossary for details)
• Inlining one-off query creation methods
• Use QueryRow where applicable for easier scanning/error handling
• Reuse defined list of columns between queries and scanners to ensure they always agree
• Make all integer types explicitly agree with the database integer types (integer/serial -> int32 and bigint/bigserial -> int64)
• Always close *sql.Rows in the function where it was created
• Add comments
• Removed named return parameters
• Remove predeclared variables where not needed
• Add some comments (probably could still use more, particularly with the info in the glossary below)

Glossary:

• Trigger: an event that "triggers" a set of code monitor actions. Currently our only trigger is a QueryTrigger.
• Action: a response to a trigger event. Currently, our only action is EmailAction, but this will be expanded very soon to include Webhook and SlackWebhook.
• Job: an asynchronously scheduled task that is executed by a worker. In the context of code monitors, this will be either a trigger job (execute a search and see if there are results) or an action job (send an email, execute a webhook, etc.)
• TriggerJob: an execution of a trigger check to see if there are any new events. In this case, all trigger jobs are search executions.
• ActionJob: an execution of a configured Action. Currently, only "send an email".
• QueryTrigger: a Trigger that checks if the results of a commit or diff search returns any new results
• EmailAction: an Action that sends an email to the configured recipient in response to an event from a Trigger

Review notes:

• Each commit is small, self-contained, and (I think) self-explanatory, so if you'd like to review this closely, I recommend looking at it commit-by-commit.
• These changes are purely style changes. This should be no external change to behavior.

What this PR does not cover:

• Removing dependency on the graphqlbackend package
• Moving the store into enterprise/internal/database (dependent on the removal of graphqlbackend dependency)
• Removing dependency on the workerutil package
• Remove dbconn.Global in tests
• Use database now() rather than passing it in the query. This might not be reasonable depending on tests.