Check-in: Improve exposed filters and flagging language

[alisonjo315]

Follow-up to #492

View

• Add either a reset button or a link to the view URL (which would feel like a reset button) so users can "clear filters."
• Add "show non-checked-in" filter.
• IF IT DOESN'T DISRUPT THE CHECK-IN FLAG, disable Ajax for this view. (If doing so is problematic for the ajax used for the Check-in Flag, please don't bother working to make it work anyway, just leave ajax enabled :) and maybe just enable "remember last selection" for the exposed filters.)
• View title: Change to lowercase "i" -- "Check-in" (wouldn't bother if weren't already tweaking the view)

Flag

• Change "registered" link text -- OR, b/c I can't think of better link text haha, change the flag/unflag link "descriptions" and "flagged/unflagged" messages:
  • Flag link description: "Click to check-in this attendee." (or "Click to check-in")
  • Flagged message: "Attendee is checked-in."
  • Unflag link text: "Checked-in" (just a typo fix, adding hyphen -- wouldn't bother if weren't already tweaking the flag)
  • Unflag description: "Click to reverse check-in (i.e. un-check-in)."
  • Unflagged message: "Attendee is NO LONGER checked-in."