fix set thumbnail and remove map

[boney-bun]

partially fix #207

the working sample of removing map:

[codecov]

Codecov Report

Merging #350 into master-qgis_server will decrease coverage by 0.04%.
The diff coverage is 21.62%.

@@                  Coverage Diff                   @@
##           master-qgis_server     #350      +/-   ##
======================================================
- Coverage                49.1%   49.05%   -0.05%     
======================================================
  Files                     267      267              
  Lines                   20342    20379      +37     
  Branches                 2626     2631       +5     
======================================================
+ Hits                     9989     9997       +8     
- Misses                   9740     9769      +29     
  Partials                  613      613

Impacted Files	Coverage Δ
geonode/maps/urls.py	96.29% <100%> (+0.46%)	⬆️
geonode/maps/qgis_server_views.py	50.17% <14.7%> (-4.73%)	⬇️

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