Add maintainer field to plugin update

[Xpirix]

• This PR is for the this issue: #303

Changes summary:

• Replace all Owner appearances with Collaborators, to be consistent with: https://github.com/qgis/QGIS-Django/pull/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 :warning:
qgis-app/plugins/forms.py	12.50%	7 Missing :warning:
...s-app/plugins/migrations/0005_plugin_maintainer.py	81.81%	2 Missing :warning:
qgis-app/plugins/models.py	50.00%	2 Missing :warning:

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Additional details and impacted files

```diff @@ 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 :+1: . Let's see what others think.

[gacarrillor]

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