Re-enable warning about banned providers in activity stream

This issue should be resolved after: https://ecosystem.atlassian.net/browse/STRM-2140

Currently we are not showing list of banned providers, because we don't have information whether they are failing due to lack of activity stream capabilities or not.

Once is done this should be a straight forward fix, just revert this commit: https://bitbucket.org/atlassian/atlassian-streams/commits/2320d28b383996e05509c548bc5180fb61f5df38/