Add geojson suppport

[boney-bun]

fix #405

[codecov[bot]]

Codecov Report

Merging #467 into 2.8.x-qgis_server will increase coverage by 0.01%. The diff coverage is 66.66%.

@@                 Coverage Diff                  @@
##           2.8.x-qgis_server    #467      +/-   ##
====================================================
+ Coverage              41.48%   41.5%   +0.01%     
====================================================
  Files                    403     403              
  Lines                  28853   28867      +14     
  Branches                3665    3666       +1     
====================================================
+ Hits                   11969   11980      +11     
- Misses                 16140   16142       +2     
- Partials                 744     745       +1

Impacted Files	Coverage Δ
geonode/qgis_server/models.py	81.95% <ø> (ø)	:arrow_up:
geonode/layers/utils.py	58.76% <0%> (ø)	:arrow_up:
geonode/layers/forms.py	80.11% <0%> (-1.44%)	:arrow_down:
geonode/layers/models.py	74.9% <100%> (+0.09%)	:arrow_up:
geonode/layers/tests.py	97.58% <100%> (+0.03%)	:arrow_up:

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[boney-bun]

thanks @myarjunar @lucernae i'll merge this. let's bring it to the testing