Make EM Bright text more flexible based on given classifications; fixes #130

This MR adds some flexibility to the output of EM bright text, where it only shows the relevant sentence if the data product as been provided (e.g. sentence on HasNS only appears if that field is in the Classifications section of the VOEvent). This will allow the circular to avoid failing if one pipeline/search does or does not upload certain fields into the future, similarly to how we treat the text in p_astro.