Add some overrides for Admin interfaces

[lucernae]

• Set default backup directory from settings
• Pass backup model id to backup process
• Override Layer Admin interface for QGIS Server backend

[lucernae]

Minor update to #506 related with kartoza/docker-geosafe#121

[codecov[bot]]

Codecov Report

Merging #509 into 2.8.x-qgis_server will increase coverage by 0.01%. The diff coverage is 54.83%.

@@                  Coverage Diff                  @@
##           2.8.x-qgis_server     #509      +/-   ##
=====================================================
+ Coverage               40.9%   40.92%   +0.01%     
=====================================================
  Files                    408      410       +2     
  Lines                  29299    29330      +31     
  Branches                3723     3723              
=====================================================
+ Hits                   11986    12003      +17     
- Misses                 16567    16581      +14     
  Partials                 746      746

Impacted Files	Coverage Δ
geonode/qgis_server/admin/admin.py	100% <ø> (ø)
...onode/base/management/commands/backup_geoserver.py	0% <0%> (ø)	:arrow_up:
...s_server/management/commands/backup_qgis_server.py	0% <0%> (ø)	:arrow_up:
geonode/qgis_server/admin/__init__.py	100% <100%> (ø)
geonode/base/admin.py	68.65% <100%> (+0.47%)	:arrow_up:
geonode/settings.py	84.96% <100%> (+0.05%)	:arrow_up:
geonode/qgis_server/admin/overrides.py	100% <100%> (ø)

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