Feature: Add lesson picker

[Deadpikle]

Allows user to choose which lessons they want to do next. Based on feature from WK website. Lesson picker only shows up if user has available lessons and hasn't hit their apprentice count. Separates lessons by level and lesson type (radical vs kanji vs vocab).

Made some minor tweaks to the SubjectModelItem class to support using those types of cells with checkmarks. Added a new LessonPickerViewController. Other than that, pretty standard minor changes (new segues, main menu table cell item, etc.).

Note: Does not currently perform checks to see if user has selected more lessons to do than (current apprentice count + number of lessons selected to do).

[Deadpikle]

Added # of lessons picked to the Begin button so that you can see how many you've picked. (The WK website has the same.)