Parent issue: #141

Add a URL prefix to all API urls:

Current URLs:

• /api/key/:seconds_valid_or_key
• /api/keys/reload
• /api/stats
• /api/torrent/:info_hash
• /api/torrents
• /api/whitelist/:info_hash
• /api/whitelist/reload

New URLs:

• /api/v1/key/:seconds_valid_or_key
• /api/v1/keys/reload
• /api/v1/stats
• /api/v1/torrent/:info_hash
• /api/v1/api/torrents
• /api/v1/whitelist/:info_hash
• /api/v1/whitelist/reload

We will keep the current ones (without prefixes) until we update the torrust index backend.