api - Update links in data to improve context-independent browsing

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//
• /api/1/content//raw/
• /api/1/release/<sha1_git>/

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Migrated from T254 (view on Phabricator)