Add maintainer field to plugin update

[Xpirix]

• This PR is for the this issue: Plugin details mixup 'created_by' 'owner' 'author' and 'maintainer' #303

Changes summary:

• Replace all Owner appearances with Collaborators, to be consistent with: Replace 'Owners' by 'Collaborators' #254
• Create a new field maintainer to store the maintainer user. It takes the value from created_by by default. This new field will be displayed as the maintainer in the plugin details.
• Create a new field display_created_by to display the created_by in the plugin details. This is set to False by default.

Please find below a GIF:

[codecov-commenter]

Codecov Report

Attention: 39 lines in your changes are missing coverage. Please review.

Comparison is base (6fe3124) 82.01% compared to head (a8db7d7) 36.97%.

Files	Patch %	Lines
qgis-app/plugins/tests/test_change_maintainer.py	36.36%	28 Missing ⚠️
qgis-app/plugins/forms.py	12.50%	7 Missing ⚠️
...s-app/plugins/migrations/0005_plugin_maintainer.py	81.81%	2 Missing ⚠️
qgis-app/plugins/models.py	50.00%	2 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff             @@
##           master     #316       +/-   ##
===========================================
- Coverage   82.01%   36.97%   -45.04%     
===========================================
  Files         139      106       -33     
  Lines        5427     4500      -927     
===========================================
- Hits         4451     1664     -2787     
- Misses        976     2836     +1860     

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[gacarrillor]

Thank you, I think it's much clearer now how to set the maintainer 👍 . Let's see what others think.

[gacarrillor]

Just to mention that this PR will fix both #260 and #303.