fix: make emails configurable

.env.test

@@ -7,3 +7,4 @@ POSTGRES_PORT=54320
 POSTGRES_USER=root
 SLACK_BOT_USER_O_AUTH_ACCESS_TOKEN=SLACK_BOT_USER_O_AUTH_ACCESS_TOKEN
 SLACK_SIGNING_SECRET=SLACK_SIGNING_SECRET
+EMAIL_PATTERNS=@manomano.com,@prt.manomano.com

README.md

@@ -202,6 +202,12 @@ Create a `.env` file containing the following variables:
 
     ![OAuth token](docs/assets/slack/oauth-token.png)
 
+- `EMAIL_PATTERNS`
+  Email patterns of your organization (separated by comma),
+  for instance `@my.org,@contractors.my.org`.
+  Used to find Slack users from Gitlab username.
+  Note that this means your org has to use the same naming scheme for emails and gitlab username.
+
 If you want Homer to connect to an **external PostgreSQL database**, you can set
 the following variables:
 

src/core/services/slack.ts

@@ -8,6 +8,7 @@ import { logger } from './logger';
 const SLACK_BOT_USER_O_AUTH_ACCESS_TOKEN = getEnvVariable(
   'SLACK_BOT_USER_O_AUTH_ACCESS_TOKEN'
 );
+const EMAIL_PATTERNS = getEnvVariable('EMAIL_PATTERNS');
 
 // This client should be used for everything else.
 export const slackBotWebClient = new WebClient(
@@ -94,10 +95,10 @@ export async function fetchSlackUserFromGitlabUser({
 export async function fetchSlackUserFromGitlabUsername(
   username: string
 ): Promise<SlackUser | undefined> {
-  return fetchSlackUserFromEmails([
-    `${username}@manomano.com`,
-    `${username}@prt.manomano.com`,
-  ]);
+  const emails = EMAIL_PATTERNS.split(',').map(
+    (emailPattern) => `${username}${emailPattern}`
+  );
+  return fetchSlackUserFromEmails(emails);
 }
 
 export async function fetchSlackUserFromId(