Fix cached translations of public_trial options:

In ba887a15 (PR #308) weights previously used for the restricted vocabulary
were dropped from PUBLIC_TRIAL_OPTIONS. Therefore the get_translated_public_trial_values
function needs to be adapted accordingly.
Move language_cache_key to opengever.base.utils.
Move `get_translated_public_trial_values` to og.base.classification and

rename it to `translated_public_trial_terms`.
Journalize any changes to `public_trial` field.