Simplify exercise editing

[elbotho]

Discussion with Wolfgang und David:

• always open complete exercise collections in editor
• (only have one edit link)

Thinks to consider/change:

• maybe offer the option to fold/unfold subexercises in the editor
• how to we merge the activity log etc?
• does the new diff view work well enough to handle collections with long answers or many subexercises?

[CarolinJaser]

Moved to Google Doc https://docs.google.com/document/d/1rSkFo2JtsSwHHsbosQcgAN4EzeePXLnqi_eCFpUM3u4/edit#