Improve default rolling retention/option data on rollover

[ghachey]

When selecting students to Retain in the Rolling Retention/Option just before a rollover all work as expected. The students gets retained after the rollover and displayed as such in his/her enrollment history. All good so far. But it keeps the Rolling Retention/Option to "Retain" in the new school year. The more likely behavior (and thus a better default) would be to set back to the more common "Next grade at current school" since the student is unlikely to Retain a second year in a row. Leaving this to Retain will almost certainly result in repeating students inadvertently.