Skill Books Tweaks

Completely removes the need for the Professorial trait (from completing the Renaissance Aspiration) to write skill books.

Tweaks:

• Best-sellers now require level 7 in both Writing and the target skill.
• The target skill level now also affects the book's quality.
• To write a skill book, you'll need level 2 in the target skill and level 2 in Writing. (This way, it stays locked if you never practiced the target skill.)