Google Drive backend for remote storage

[sergei-maertens]

• Get service accounts working
• Make backends pluggable/configurable via config file (possibilty for S3, Azure, rsync... as backends)
  • Backend specific config
  • Optional dependencies
• Figure out if a backup was transferred already or not
• Upload backups to Drive
• Backup rotation (too dangerous for now)
• (Override) flags/options: not MVP
  • Overwrite backup
  • Specify (single) backup to transfer
• Pipe upload after backup creation (= no separate cronjob needed)

[Bonemind]

Seems to break CI, needs fixing :)

[codecov-io]

Codecov Report

Merging #2 into develop will increase coverage by 1.81%.
The diff coverage is 91.35%.

@@           Coverage Diff            @@
##           develop    #2      +/-   ##
========================================
+ Coverage    89.18%   91%   +1.81%     
========================================
  Files            5    13       +8     
  Lines          379   600     +221     
  Branches        49    77      +28     
========================================
+ Hits           338   546     +208     
- Misses          29    38       +9     
- Partials        12    16       +4

Impacted Files	Coverage Δ
ctrl_z/transfer/archive.py	100% <100%> (ø)
ctrl_z/transfer/__init__.py	100% <100%> (ø)
ctrl_z/utils.py	100% <100%> (ø)
ctrl_z/constants.py	100% <100%> (ø)
ctrl_z/backup.py	88.16% <100%> (ø)	⬆️
ctrl_z/retention.py	100% <100%> (ø)	⬆️
ctrl_z/transfer/backends/__init__.py	100% <100%> (ø)
ctrl_z/transfer/transfer.py	100% <100%> (ø)
ctrl_z/config.py	91.66% <66.66%> (ø)	⬆️
ctrl_z/transfer/backends/base.py	83.33% <83.33%> (ø)
... and 11 more

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Travis build fixed but needs tests for new feature(s) ;)

[sergei-maertens]

The whole integration with team drives is still a bit unclear, but we'll work it out in the coming weeks.