fix: the user is not correctly considered as active (#280)

**Relative Issues:** Resolve #279 

**Describe the pull request**
One column on the model user called `is_a_user` must be set as true when
an User takes the action to join the app. But actually, the number of
active users seems to lower. This PR fix this issue

**Checklist**

- [x] I have linked the relative issue to this pull request
- [x] I have made the modifications or added tests related to my PR
- [x] I have added/updated the documentation for my RP
- [x] I put my PR in Ready for Review only when all the checklist is
checked

**Breaking changes ?**
no

**Additional context**
A manual migration in the database will be perfromed to set all user
with duo account linked as active.

internal/api/account_callbacks.go

@@ -35,14 +35,31 @@ func accountLinkCallback(ctx context.Context, db *modelgen.Client, account *mode
 	case string(typesgen.ProviderGithub):
 		githubLinkCallback(bgCtx, db, account)
 	case string(typesgen.ProviderDuo):
-		break
+		duoLinkCallback(bgCtx, db, account)
 	default:
 		log.Warn().Str("accountType", account.Type).Msg("no callback is defined")
 	}
 }
 
+// duoLinkCallback is the callback for 42 (DUO) accounts when the account is
+// linked to an user. Only happened when an User will connect itself for the
+// first time
+func duoLinkCallback(ctx context.Context, db *modelgen.Client, account *modelgen.Account) {
+	user, err := account.User(ctx)
+	if err != nil {
+		log.Error().Err(err).Msg("failed to retrieve user")
+		sentry.CaptureException(err)
+		return
+	}
+
+	if err := db.User.UpdateOne(user).SetIsAUser(true).Exec(ctx); err != nil {
+		log.Error().Err(err).Msg("failed to update user")
+		sentry.CaptureException(err)
+	}
+}
+
 // discordLinkCallback is the callback for Discord accounts when the account is
-// linked to a user. It will add the user to the Discord server.
+// linked to an user. It will add the user to the Discord server.
 func discordLinkCallback(ctx context.Context, db *modelgen.Client, account *modelgen.Account) {
 	// Invite the user to the S42 server using the Discord API
 	err := discord.
@@ -54,7 +71,7 @@ func discordLinkCallback(ctx context.Context, db *modelgen.Client, account *mode
 }
 
 // githubLinkCallback is the callback for Github accounts when the account is
-// linked to a user. It will follow the creator of the repository and star
+// linked to an user. It will follow the creator of the repository and star
 // the repository.
 func githubLinkCallback(ctx context.Context, db *modelgen.Client, account *modelgen.Account) {
 	client := github.NewClient(oauth2.NewClient(ctx, oauth2.StaticTokenSource(