API overhaul: add version prefix `v1` to the API endpoints

[josecelano]

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.

[josecelano]

We know that using a hardcoded API version on the URL is not a good practice because it forces you to rewrite all URLs when you want to upgrade the API client.

This is a temporary change that will allow us in the future to implement a new API without affecting current consumers. The current consumers only have to add the prefix now to continue using the API instead of changing their request to include a version value in the header, which is our preferred way to implement versioning in the new major API change.

See GitHub API versioning for an example of how to implement API versions.

cc @da2ce7 @WarmBeer