Simplify Canvas group import and export

[wynnset]

• Add new separate screens for importing and exportin Canvas groups with minimal controls
• Add new links in the Course homepage menu to the 2 new pages
• Group all Canvas functionality into 1 new submenu on Course homepage
• Comment out link to original Canvas sync screen with fine-tune functionality
• When importing, always import users from Canvas and completely replace iPeer groups with Canvas groups
• When exporting, always replace groups in Canvas with iPeer groups if they have the same name

Fixes #558

[wynnset]

@kitsook I've made all those changes. Would you mind reviewing again? Thanks!