Add support for triggering builds on Jenkins from the new frontend

Slots to be built (from the "Start slots" view) are prepared an added to the database with a special flag telling that they have to be built.

The Jenkins job https://jenkins-lhcb-nightlies.web.cern.ch/job/nightly-builds/job/new-slots-build-poll/ regularly (currently every 2 minutes) checks if there are new slots to build (using the result of a CouchDB view). If so, the special flag is removed and we trigger a checkout job (entry point to trigger the whole build in Jenkins).

I added a placeholder function that can be used to dispatch the slots to the new or the legacy system. For the moment it only uses the legacy one.

Closes #37 (closed)