refactor constants from ObservationService to DTO and use contants
instead of names in observationDTO
Merge branch 'master' of https://github.com/mpi2/PhenotypeArchive

* 'master' of https://github.com/mpi2/PhenotypeArchive:
  Added disease panel to dropdown navigation bar. Fixed weird div spacing issue where disease panel is squashed against the preceeding panel.