setup pilots_compare user preference (default it to an empty arrayref, or split the user's choices into an arrayref)
check for pilots_compare user pref as an arrayref already
collect user's pilots' trained levels for current skill's pre-requisites
add a column for each pilot in the provided list, showing the state of their training of pre-requisite skills
fix warning when pilot doesn't have skill trained at all
fix warning for untrained skills in prerequisites table template
common method for removing pilots from the user preference pilots_compare
add dropdown to pilot icon on skill prereqs table to provide access to pilots_compare remove method
move the required skill's rank underneath the add-pilot dropdown to save a little space
simplify removing a pilot from the pilot_compare list just a touch
close up a hole that would have allowed someone to add another person's pilot to the pilot_compare pref, thus exposing other users' trained skills
use the correct pilot_id for the removal link
mark primary/secondary/tertiary rows in skill prereqs with a new section class
feeble attempt at styling table section rows in a way to more clearly demarcate the groups -- feeble in that Foundation's builtin row background alternating has no knowledge of this, so the non-section header rows don't color consistently