Fix fake user creation based on username rules (#179)

Author: andrecasal

tests/e2e/onboarding.test.ts

@@ -17,7 +17,10 @@ function extractUrl(text: string) {
 test('onboarding with link', async ({ page }) => {
 	const firstName = faker.person.firstName()
 	const lastName = faker.person.lastName()
-	const username = faker.internet.userName({ firstName, lastName }).slice(0, 15)
+	const username = faker.internet
+		.userName({ firstName, lastName })
+		.slice(0, 15)
+		.replace(/[^a-z0-9_]/g, '_')
 	const onboardingData = {
 		name: `${firstName} ${lastName}`,
 		username,