Clarify that some fields are recent presentation.

Some of the fields in solr we populate with the most recent presentation, make it clear that this is what we're doing.
WL-2902 Hide courses older than 2 years.

If a course hasn't run in the last 2 years then we don't display it in the search results.
WL-2912 Add the tests.

This should have been part of the earlier commit.