Restructure the public alerts page

Tracking changes to the public alerts page. This is more a restructuring than a tear-down because I think I added enough flexibility with the last update to implement changes pretty quickly:

• make superevents/public/{O3, ER15, O4}/ separate pages for each run (#303 (comment 719265))
  • if a user tries to go to a page that isn't in the run list then return 404?
• redirect superevents/public/ to the most recent observation period (in this case, O4)
• define a significant/insignificant filter criteria for each run (or, O3 vs ER15/O4).
  • brain dump: each run can have its own set of Q() filters. If an event matches the criteria in the view loop, then flag it as insignificant and
• Have a button to show/hide insignificant events? I'm thinking a bootstrappy button like the one that shows/hides logs at the moment. If there is no filter criteria, then don't show the button.
• Retracted events have the row background in red. insignificant events' rows have a... grey? background?

Out of scope for this MR, but included anyway:

• consistent bootstrap container behavior across pages (note: this fixes mobile views by keeping the hamburger menu fixed at the top right of the screen, and the child container overflows if the content is too wide).
• the bottom row attribution images stay in a row and resize with screen
• general javascript and css cleanup.