Add localizable labels to instructor charts

[bmtcril]

Per #169 we should add the following (where it makes sense) to all instructor dashboard charts:

• Descriptive title
• Description
• X axis label
• Y axis label
• Localized legend (ex course name, problem name, etc) or turn it off

Here are the current charts to work on, future charts should include these as part of being added.

• Learners per day
• Learners by enrollment mode
• Enrollment events per day
• Problem engagement overview
• Grade distribution
• Response distribution
• Grade distribution
• Attempts to first success
• Hints and answers
• Watches per video
• Transcripts
• Video drilldown

[bmtcril]

Remaining issues:

• Enrollments By Enrollment Mode uses the verb name in tooltip display "audit" / "verified"
• Enrollment Events Per Day uses the verb name in tooltip display "registered" / "unregistered"
• Problem Responses - displays "True" / "False" for correct / incorrect responses
• Watches Per Video - displays "Total plays" / "Unique watchers" from the metrics
• Several places where "count" appears in the tooltips, I'm not sure if we want to turn this off or try to rename it