bitly integration: upgrade to API v4

[examknow]

Bitly will be killing version 3 of their API (the one currently used by BitBot) at the end of this month. We'll need to update BitBot to use version 4 in order to keep support for Bitly. The update process seems fairly straightforward and should be a pretty simple fix.

[examknow]

From what I understand, all we need to do in our case is send the API key in a bearer token header instead of in the GET params.