Drop 'src/course' folder in favour of 'build/course'

[oliverfoster]

Advantages

• makes src folder "code only"
• unifies build pathways across AAT and Fwk
• deduplicates course assets when stored on disk
• allows developers to concentrate on build output
• simplifies grunt code

Suggested timeline

Release with v6

[tomgreenfield]

Please also include cleaning of build folder.

Reference: adaptlearning/adapt_authoring#2235.

Edited: grunt clean does this ref

[guywillis]

👍