Simplify api browsing by having links to specific entities where it makes sense:
- A release can target other entities (including revision but not limited to... indeed the target's type can vary)
- Directory targets either content or directory.
- Revision targets directory
- Content `targets` raw content
api impacted:
- /api/1/directory/<sha1_git>/
- /api/1/revision/<sha1_git>/
- /api/1/revision/<sha1_git>/log/
- /api/1/content/<q>/
- /api/1/content/<q>/raw/
- /api/1/release/<sha1_git>/