Fix #26: Add assets management

[nikochiko]

Fixes #26

• Add assets table in database and helper to save file
• Serve assets from app at /assets/
• Modify CourseLoader to save instructor photo as static asset
• Add helper to CLI to setup assets directory
• Fix template typo: instructors section in course.html

[nikochiko]

Merging this now, but have branched out a review suggestion into #29 for further consideration.