DCOR-Aid 0.7.0

• feat: implement download manager
• feat: rename first tab to "Find Data" and allow searching for
  private datasets
• fix: perform sanity checks for datasets (#27)
• enh: allow to download all resources of a collection (#13)
• enh: implement search for packages the user has access to
• enh: implement DBInterrogator functions for generating a private
  user db (partly #5)
• setup: bump dclab from 0.35.3 to 0.35.6
• ref: simplify dbmodel submodule (just use Interrogator)
• ref: expand dedicated api submodule (moved functions from
  upload.dataset)
• ref: introduce dedicated common submodule
• ref: introduce dedicated worker submodule for KThread andDaemon