E2E test: download a torrent with a personal announce URL

[josecelano]

Parent issue: #148
Relates to:

• authentication error: missing authentication key param for private tracker #412
• authentication error: missing authentication key param for private tracker #412 (comment)

When the user is logged in and downloads a torrent, the announce URL should have the user's tracker key.

We should first define what it's the right behavior and if we need to change the actual behavior. See #412 (comment)

[josecelano]

I decided not to check that the downloaded torrent contains the tracker keys because there is no direct way to do it with Cypress. We have to parse the downloaded torrent and also check that the tracker key is the expected key. Finally, I have checked that the app works in "private" mode, but checking the private keys in the torrent file is something we could do in the Index.