Resolve "Add CouchDB view to list slots including a MR"

I already pushed the view to CouchDB, they are indexing at the moment.

Note that I added 2 views:

• slots for a given MR (slightly modified from the suggestion in #41 (closed))
  • https://lhcb-couchdb.cern.ch/nightlies-nightly/_design/merge_requests/_view/mrs?key=%5B%22LHCb%22%2C2264%5D
• mr slots for a given ref slot
  • https://lhcb-couchdb.cern.ch/nightlies-nightly/_design/merge_requests/_view/mr_slots_by_ref_slot?key=%5B%22lhcb-master-ref%22%2C216%5D

I also removed an obsolete view and added an index I need for lb-nightly-builds-frontend#23 (closed).

Closes #41 (closed)