Improvements to the pre-computed pop sizing UI on Audience page

[mikewilli]

Right now, it's mostly invisible (somewhat intentionally) unless you know the targeting combinations that will make it appear. We should work to make this more visible. A few options (worth thinking if there are other/better options too):

• don't hide the population sizing behind the current parameters
  • allow separate selection of sizing info (instead of selecting all the parameters via audience/targeting, select the specific sizing combo from the list of pre-computed options)
  • collapse all sizing info by default (mostly because it takes up a lot of the screen)
• show a note indicating why sizing info is not displayed
  • something like "Sorry, no pre-computed sizing info is available for the selected targeting."
  • possible including info about what targeting has pre-computed sizing available

┆Issue is synchronized with this Jira Task