expose donation stats to a slack slash-command

[echarlie]

since slack can talk to authenticated endpoints, we should have expose donation stats to it---the same stats we display prominently on /admin/donations

[echarlie]

This is similar to what was done for our latest track slack command https://github.com/wuvt/wuvt-site/blob/master/wuvt/trackman/views.py#L96 however should require an auth key from slack.

[mutantmonkey]

We'd probably want to implement this as part of https://github.com/wuvt/slack-hooks

[echarlie]

okay, so to implement this, donation stats need to be exposed via an API call, and the rest can be handled in slack-hooks if wanted.