Warn users about potentially breaking Anaconda's new TOS

[ericdill]

As conda-mirror maintainers, we should probably consider warning users that they may be breaking Anaconda's new terms of service for repo.anaconda.com unless either of the following is true:

1. They have an existing commercial relationship with Anaconda
2. They are not a large-scale commercial entity, specifically referring to this line in the new TOS, we are not granting you permission to use the Repository for large-scale commercial activities, such as downloading or mirroring the entire Repository or use of the Repository by multiple members of the business you work for.

I've got a draft PR that implements what I think is a reasonable message, though I'm sure we will need to update the language based on folks opinions.

Before we get to the specific language in the warning / error message though, is anyone opposed to forcing users to address the TOS changes in conda-mirror itself *if they are using Anaconda's channels on repo.anaconda.com or repo.continuum.io?

[wagnerpeer]

I would also welcome the idea of adding support for people who already are in a commercial relationship with Anaconda.
This would basically mean to incorporate a security token to access private/commercial channels.

[analog-cbarber]

Perhaps the relevant Anaconda channels should be added to a default forbidden channel list that the user must
explicitly override in order to download from those channels.