Refactor GeoNode backup admin command

[lucernae]

• Add command for GeoNode backup for QGIS Server backend
• Refactor default management command and Admin command to execute backup/restore command based on OGC backend being used

[lucernae]

Related with kartoza/docker-geosafe#121

[codecov[bot]]

Codecov Report

Merging #506 into 2.8.x-qgis_server will decrease coverage by 0.46%. The diff coverage is 0%.

@@                 Coverage Diff                  @@
##           2.8.x-qgis_server    #506      +/-   ##
====================================================
- Coverage              41.37%   40.9%   -0.47%     
====================================================
  Files                    403     408       +5     
  Lines                  28968   29299     +331     
  Branches                3678    3723      +45     
====================================================
  Hits                   11986   11986              
- Misses                 16236   16567     +331     
  Partials                 746     746

Impacted Files	Coverage Δ
geonode/utils.py	48.76% <0%> (-0.14%)	:arrow_down:
..._server/management/commands/restore_qgis_server.py	0% <0%> (ø)
...node/base/management/commands/restore_geoserver.py	0% <0%> (ø)
geonode/base/management/commands/restore.py	0% <0%> (ø)	:arrow_up:
...s_server/management/commands/backup_qgis_server.py	0% <0%> (ø)
geonode/base/management/commands/backup.py	0% <0%> (ø)	:arrow_up:
...onode/base/management/commands/backup_geoserver.py	0% <0%> (ø)
geonode/qgis_server/management/commands/helpers.py	0% <0%> (ø)
... and 3 more

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