Add download zipped files for non-shapefile.

[ismailsunni]

This is a follow up from PR #232

[codecov]

Codecov Report

Merging #239 into master-qgis_server will increase coverage by 4.43%.
The diff coverage is 25%.

@@                  Coverage Diff                  @@
##           master-qgis_server    #239      +/-   ##
=====================================================
+ Coverage               40.26%   44.7%   +4.43%     
=====================================================
  Files                     253     254       +1     
  Lines                   18317   18334      +17     
  Branches                 2411    2414       +3     
=====================================================
+ Hits                     7376    8196     +820     
+ Misses                  10475    9663     -812     
- Partials                  466     475       +9

Impacted Files	Coverage Δ
geonode/qgis_server/urls.py	100% <ø> (ø)	⬆️
geonode/settings.py	90.61% <ø> (+3.28%)	⬆️
geonode/tests/integration.py	0% <0%> (ø)	⬆️
geonode/qgis_server/signals.py	85.51% <57.14%> (-1.44%)	⬇️
geonode/layers/metadata.py	12% <0%> (ø)	⬆️
geonode/contrib/geosites/local_settings.py	0% <0%> (ø)
geonode/maps/tests.py	98.24% <0%> (+0.58%)	⬆️
geonode/geoserver/signals.py	13.96% <0%> (+1.11%)	⬆️
geonode/maps/utils.py	40% <0%> (+1.53%)	⬆️
geonode/maps/models.py	67.08% <0%> (+1.66%)	⬆️
... and 31 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update cf6883b...8406044. Read the comment docs.

[lucernae]

LGTM. Could you add a unittest, to make sure we have all the links needed?