static async loadDomainMappingList() {
        StatusPage.domainMappingList = await R.getAssoc(`
            SELECT domain, slug
            FROM status_page, status_page_cname
            WHERE status_page.id = status_page_cname.status_page_id
        `);
    }
Loads domain mapping from DB
Return object like this: { "test-uptime.kuma.pet": "default" }
@returns {Promise<void>}