Fix logic for education of very dumb slaves and deeducation of very smart ones.  Now, the smarter they are, the faster they learn and the slower they unlearn, and the bonus always improves their chance of learning or unlearning depending on the goal of the schoolroom.  Added comments to make it clearer what's going on.

This reverts merge request !5713

Refactor speechLessons a little to reduce duplication, and differentiate in text between learning a language to accent level 3 and accent level 2 in language-upgrading schoolrooms.