Address 'on the fly' changes made to S190408an

A number of changes were made on the fly for S190408an: https://gcn.gsfc.nasa.gov/gcn3/24069.gcn3 compared to the original: https://gracedb.ligo.org/apiweb/superevents/S190408an/files/preliminary-circular.txt,0

We may want to adjust the automatic templates based on these changes.

• The first paragraph received a major overall change in language
• In the bayestar section, 'event' was changed to 'candidate'
• The em_bright paragraph has slight language changes
• Additional explanation was given to the distance; (a posteriori mean +/- standard deviation)
• Added SPIIR reference to Qi Chu's Thesis