add management command to prune couch views

it occurred to me as i've been waiting for the india server views to reindex that many shouldn't have existed any more.

all this work we're doing to delete couch views and in many cases we're not actually deleting them!

depends on https://github.com/dimagi/dimagi-utils/pull/243 and codependent with https://github.com/dimagi/fluff/pull/96
F401 'settings' imported but unused