better validate attendance_record to unique user_id per day

The old validation required instantiating the associated user model,
then all of its other attendance records and making sure the created_at
doesn't match the current day. This created a lot of overhead for what
should be a simple query. The new way uses a condition to scope the
uniqueness check on user_id to just records for today.
fix syntax on lambda
changed the way flash messages are rendered

We needed to add an html_safe segment to the flash message. The best
solution was to have the flash helper accept the block, in which we
pass the flashes we want to render. This allows us to call html_safe
on the parts of the flash we want to be html safe.
pass attendance record destroy link through secure flash segment
refactor attendance_records/index view
refactor routes.rb
fix spelling error
remove before blocks to make tests more readable