Revamp Public Alerts Page for O4

Reference: #267 (closed)

• remove /O3/ from /superevents/public/O3/ url path
• have /O{N}/ redirect to /superevents/public/O3/
• define ER15 and O4 in RUN_MAP (TODO: fix that with actual gpstimes when they are finalized)
• have SupereventPublicView loop over observation runs, make one table for each
• figure out source properties from new VOEvent entries (like HasMassGap)
• should source properties come from p_astro instead?
• clean up page and tables
• add links to to go to specific part of page? automatically forwarded from /O3/, /O4, etc?
• A pull-down menu from the "Public Alerts" banner link?