Fix and optimize API for sandbox list

The sandbox_for method was previously returning an array of identical
items because all items pushed into the output array were linked to the
same sandbox hash.
Add user's sandbox list to course form
Deleting trailing ':::course' from cross-list sis_source_id

Not needed or being used
Merge pull request #106 from va7map/canvas212-sandbox

Add sandbox list to Start a New Course form
Merge pull request #107 from ronvs/clean-sis-source-id-for-crosslist

Deleting trailing ':::course' from cross-list sis_source_id
Tweak button layout + adopt some built-in styles
Add help button to Start a New Course form
Merge pull request #108 from va7map/canvas212-tweaks

Layout tweaks + Help button (Start a New Course)