Add `SkillMasteryHelper.kt` and `getFamiliarWordReadingMastery()`

[jo-elimu]

Calculate a student's mastery (percentage) of the LitercySkill.FAMILIAR_WORD_READING.

I suggest we rename MasteryHelper.kt to ContentMasteryHelper.kt, and introduce a new utility class for keeping track of a student's evolving skill mastery scores: SkillMasteryHelper.kt.

Originally posted by @jo-elimu in #229