Fix qgis server bound projection

[lucernae]

Fix bbox projection for leaflet in QGIS Server

• Let GeoServer and QGIS Server handle misprojection issues if GeoNode can't handle it
• QGIS Server will always transform bbox to EPSG 4326 to work consistently with leaflet
• QGIS Server thumbnail request will try to maintain aspect ratios of the
  requested extent to avoid squashed image

Notes

• Most of the problem is because main geonode docker image uses GDAL 1. It can't detect most EPSG. This is fixed in: kartoza/geonode-docker@db60cae
• Even if using GDAL 2, sometimes it can't detect correct EPSG code (which is needed to draw correct bbox in leaflet). GeoServer can detect it correctly. In our case (QGIS Server) we transform the bbox to EPSG:4326 by default. The actual CRS is preserved in the file and handled by QGIS. Example layer is in gisdata package Air_Runways.shp

Issue links

• Override this PR: fix bug on srid when upload a vector layer #442 , fix thumbnails squashed #439
• Fix thumbnails on testing are squashed #374

[codecov]

Codecov Report

Merging #464 into 2.8.x-qgis_server will increase coverage by 0.29%.
The diff coverage is 85.84%.

@@                  Coverage Diff                  @@
##           2.8.x-qgis_server     #464      +/-   ##
=====================================================
+ Coverage              41.12%   41.41%   +0.29%     
=====================================================
  Files                    403      403              
  Lines                  28743    28811      +68     
  Branches                3653     3659       +6     
=====================================================
+ Hits                   11821    11933     +112     
+ Misses                 16195    16130      -65     
- Partials                 727      748      +21

Impacted Files	Coverage Δ
geonode/tests/integration.py	0% <ø> (ø)	⬆️
geonode/qgis_server/helpers.py	67.04% <100%> (+0.96%)	⬆️
geonode/qgis_server/tests/test_views.py	99.14% <100%> (+0.13%)	⬆️
geonode/qgis_server/tasks/update.py	75% <100%> (+16.93%)	⬆️
geonode/layers/utils.py	57.27% <38.46%> (+0.61%)	⬆️
geonode/qgis_server/signals.py	87.93% <73.33%> (+1.77%)	⬆️
geonode/base/models.py	74.92% <0%> (-0.29%)	⬇️
... and 1 more

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