Overhaul tracker keys in torrents and magnet links

[josecelano]

1. Tracker running in `Public` or `Whitelisted` mode. 
   Downloaded torrents should never include any tracker key.
3. Tracker running in `Private` or `PrivateListed` mode.
   2.1 Guest user. 
       Downloaded torrents should never include any tracker key.
   2.2 Authenticated user
       2.2.1 Index can get the tracker user's key (from the Index database or generate new ones).
             - We should include the user's tracker key.
             - We should change the download torrent button label from `download torrent` to `download private torrent` (this is an Idex GUI issue).
       2.2.2 The user's tracker key is expired and the tracker API is unavailable (can't generate a new one).
             Return a `503 Service Unavailable` response with the text `Can't generate user's tracker key`.

Subtasks

• Never include keys in the downloaded torrent file when the tracker is open (public or public-whitelisted).
• Never include keys in the magnet link when the tracker is open (public or public-whitelisted).
• Return an error when we can't connect to the tracker to generate the user's keys. Both for the download torrent endpoint and torrent details endpoint (magnet link).
• Return 503 instead of 500 when the tracker is offline and for example we can not generate the user's tracker keys.
• Removed duplicate URLs in the tracker lists. Both torrent file and torrent info endpoints.
• Force the main tracker URL (the one in the tracker config) to be the first in the tracker lists, both in the tracker torrent file endpoint and the torrent details endpoint.
• Panic starting the application with an invalid racker configuration. For example: private UDP tracker.

Subtasks in the Index GUI

• Rename button "download torrent" to "download private torrent" when the tracker is in close mode torrust-index-gui#486.

[josecelano]

It's not easy to test this new behavior because the current test is using the "shared" env which uses a public tracker. I should run a second env with a private tracker but I think that's too complex. I would prefer to test this functionality with unit tests. The problem is the torrust_index::services::torrent::Index service is too big. Maybe we could refactor it to a command/handler pattern.

For the time being, I will test manually and remove the E2E test for the personal announce URL.

cc @WarmBeer @da2ce7

[josecelano]

ACK 9b7c5c8