Smarter review search

[cheniel]

Review search currently only allows single words or phrases. We should support multiple words. For example, when someone searches "difficult hard", we should surface reviews that have either "difficult" or "hard" rather than only those that have "difficult hard" (probably none).

Keep in mind we use review search to surface reviews from a specific professor. Any changes to review search should not break this. We should also make sure the search cannot become too computationally expensive (possibly by limiting the number of words).