Display download bar in all cases and make the timeout configurable via an env var

[maximlt]

We have observed two issues when files are downloaded in the prepare phase:

1. Responses without Content-Length lead to no download bar being displayed at all
2. Downloads longer than 10 minutes time out

This PR addresses 1) by displaying a progress bar in any case, so the user is aware of some sort of progress. They can also see if it hangs and decide to abort if so.

2 isn't fully addressed, instead, the default timeout of 10 minutes is made configurable via the ANACONDA_PROJECT_DOWNLOADS_TIMEOUT environment variable. I guess the best approach would be to implement a timeout that is raised only when no new chunk is received after X seconds, instead of setting a global request timeout.

The current default timeout of 10 minutes could also be increased, 10 minutes is quite short when downloading big files.

[maximlt]

@AlbertDeFusco I've been told to ping you! :)