Tweak user model methods

Some of the methods (user#enrollment_for, user#instructing?) were using
the style of operating on the return value of a block, like:

def my_method
  @values.select do |value|
    value.is_true?
  end.first
end

Where it is calling the method 'first' on the return value of the
block, after the 'end' keyword.  This is valid, but having "end.first"
was messing up ctags, so I modified this in favor of:

def my_method
  @values.select { |value| value.is_true? }.first
end

Purely cosmetic, but now my ctags work right :)