Refactor deployFeed replyCallback functionality to be more reusable

[IanWoodard]

For a while we only ever had one usage of replyCallback:

eng-pipes/src/brain/gocdSlackFeeds/index.ts

Lines 236 to 293 in f4b4c07

	replyCallback: async (pipeline) => {
	const pauseCause = getPauseCause(pipeline);
	if (pauseCause == null) return [];
	const [base, head] = await getBaseAndHeadCommit(pipeline);
	const authors = head ? await getAuthors('getsentry', base, head) : [];
	// Get unique users from the authors
	const uniqueUsers = await getUniqueUsers(authors);
	// Pick at most 10 users to cc
	const ccUsers = uniqueUsers.slice(0, 10);
	const ccString = ccUsers
	.map((user) => {
	return `<@${user.slackUser}>`;
	})
	.join(' ');
	const failedJob = pipeline.stage.jobs.find(
	(job) => job.result.toLowerCase() === 'failed'
	);
	if (!failedJob) {
	// This should never happen, but if it does, we want to know about it
	Sentry.captureException(
	new Error('Failed to find failed job in failed pipeline')
	);
	return [];
	}
	const gocdLogsLink = `https://deploy.getsentry.net/go/tab/build/detail/${pipeline.name}/${pipeline.counter}/${pipeline.stage.name}/${pipeline.stage.counter}/${failedJob.name}`;
	const sentryReleaseLink = pipeline.name.includes('s4s')
	? `https://sentry-st.sentry.io/releases/backend@${head}/?project=1513938`
	: `https://sentry.sentry.io/releases/backend@${head}/?project=1`;
	const blocks = [
	header(
	plaintext(`:double_vertical_bar: ${pipeline.name} has been paused`)
	),
	section(
	markdown(`The deployment pipeline has been paused due to detected issues in ${pauseCause}. Here are the steps you should follow to address the situation:\n
	:mag_right: *Step 1: Review the Errors*\n Review the errors in the *<${gocdLogsLink}|GoCD Logs>*.\n
	:sentry: *Step 2: Check Sentry Release*\n Check the *<${sentryReleaseLink}|Sentry Release>* for any related issues.\n
	:thinking_face: *Step 3: Is a Rollback Necessary?*\nDetermine if a rollback is necessary by reviewing our *<${IS_ROLLBACK_NECESSARY_LINK}|Guidelines>*.\n
	:arrow_backward: *Step 4: Rollback Procedure*\nIf a rollback is necessary, use the *<${ROLLBACK_PLAYBOOK_LINK}|GoCD Playbook>* or *<${GOCD_USER_GUIDE_LINK}|GoCD User Guide>* to guide you.\n
	:arrow_forward: *Step 5: Unpause the Pipeline*\nWhether or not a rollback was necessary, make sure to unpause the pipeline once it is safe to do so.`)
	),
	];
	if (ccUsers.length > 0) {
	blocks.push(
	context(
	markdown(
	`cc'ing the following ${
	uniqueUsers.length > 10 ? `10 of ${uniqueUsers.length} ` : ''
	}people who have commits in this deploy:\n${ccString}`
	)
	)
	);
	}
	return blocks;
	},

, we are starting to introduce more usages of it (#790). We should refactor the deployFeed to make the flow of finding authors and pinging them in response to a failure more reusable.