feat(coderd): add company logo when available for email notifications (#14935)

This PR aims to close #14253 

We keep the default behavior using the Coder logo if there's no logo
set.
Otherwise we want to use the logo based on the URL set in appearance.

---------

Co-authored-by: defelmnq <yvincent@coder.com>

coderd/notifications/fetcher.go

@@ -0,0 +1,57 @@
+package notifications
+
+import (
+	"context"
+	"database/sql"
+	"errors"
+	"text/template"
+
+	"golang.org/x/xerrors"
+
+	"cdr.dev/slog"
+)
+
+func (n *notifier) fetchHelpers(ctx context.Context) (map[string]any, error) {
+	appName, err := n.fetchAppName(ctx)
+	if err != nil {
+		n.log.Error(ctx, "failed to fetch app name", slog.Error(err))
+		return nil, xerrors.Errorf("fetch app name: %w", err)
+	}
+	logoURL, err := n.fetchLogoURL(ctx)
+	if err != nil {
+		n.log.Error(ctx, "failed to fetch logo URL", slog.Error(err))
+		return nil, xerrors.Errorf("fetch logo URL: %w", err)
+	}
+
+	helpers := make(template.FuncMap)
+	for k, v := range n.helpers {
+		helpers[k] = v
+	}
+
+	helpers["app_name"] = func() string { return appName }
+	helpers["logo_url"] = func() string { return logoURL }
+
+	return helpers, nil
+}
+
+func (n *notifier) fetchAppName(ctx context.Context) (string, error) {
+	appName, err := n.store.GetApplicationName(ctx)
+	if err != nil {
+		if errors.Is(err, sql.ErrNoRows) {
+			return notificationsDefaultAppName, nil
+		}
+		return "", xerrors.Errorf("get application name: %w", err)
+	}
+	return appName, nil
+}
+
+func (n *notifier) fetchLogoURL(ctx context.Context) (string, error) {
+	logoURL, err := n.store.GetLogoURL(ctx)
+	if err != nil {
+		if errors.Is(err, sql.ErrNoRows) {
+			return notificationsDefaultLogoURL, nil
+		}
+		return "", xerrors.Errorf("get logo URL: %w", err)
+	}
+	return logoURL, nil
+}