internal renaming: domains (collections) and collections (groups)

[moritzschepp]

With #199, the user facing renaming was done. We still have the following aspects to align with the new names and changing that is not backwards compatible for existing scripts and api clients. We therefore add this issue to v5.

• rename mysql tables
• rename api endpoints
• rename api content (e.g. xml elements) and bump up schema version
• rename active record classes and methods (associations etc.)
• rename controllers
• rename translation keys