Create custom command to validate existing plugins on server

[sumandari]

Please see #178

This PR to validate the existing plugins on server, which has been uploaded before the metadata validation function fixed.
Custom command will send email to the creator and author of invalid plugins.

[codecov-commenter]

Codecov Report

Merging #179 (23fea87) into master (222b10d) will increase coverage by 5.44%.
The diff coverage is 88.44%.

@@            Coverage Diff             @@
##           master     #179      +/-   ##
==========================================
+ Coverage   68.21%   73.66%   +5.44%     
==========================================
  Files         102      108       +6     
  Lines        3886     4162     +276     
==========================================
+ Hits         2651     3066     +415     
+ Misses       1235     1096     -139     

Impacted Files	Coverage Δ
...s/management/commands/validate_existing_plugins.py	66.66% <66.66%> (ø)
qgis-app/plugins/views.py	30.07% <77.77%> (+11.46%)	⬆️
qgis-app/plugins/tests/test_views.py	89.55% <89.55%> (ø)
qgis-app/plugins/migrations/0002_plugininvalid.py	100.00% <100.00%> (ø)
qgis-app/plugins/models.py	81.31% <100.00%> (+20.12%)	⬆️
qgis-app/plugins/tests/model_factories.py	100.00% <100.00%> (ø)
qgis-app/plugins/tests/test_models.py	100.00% <100.00%> (ø)
...pp/plugins/tests/test_validate_existing_plugins.py	100.00% <100.00%> (ø)
qgis-app/plugins/urls.py	100.00% <100.00%> (ø)
... and 10 more

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[timlinux]

@sumandari I didnt look at the code, but it probably makes sense to only look at the latest version of each plugin - can you confirm if it does that?

[timlinux]

Also it would be nice to have view or something of plugins whose latest version fails validation so that we can keep track of which ones are still problematic as fixes come in....

[sumandari]

[sumandari]

Once we run management command, it will create invalid plugin list.
When the plugin version updated and add new plugin version(with validation mechanism upon upload), it will be removed from invalid plugin list.

[timlinux]

LGTM thanks @sumandari !