Check that all options in QGIS Server tabs are correct

[qgib]

Author Name: Etienne Trimaille (@Gustry)
Original Redmine Issue: 17883
Affected QGIS version: master
Redmine category:qgis_server

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Since the QGIS Server refactoring is done, it would be good to check if all QGIS Server settings in the Layer Panel are still correct and used.
I'm referring to the "Legend URL", "Metadata URL" and other fields.
Maybe we should add some tooltips, it's not clear if the legend URL will replace the default generated legend in a GetLegendGraphics request.

Based on a discussion in Nødebo and in Gitter with Alessandro.

[qgib]

Author Name: Giovanni Manghi (@gioman)

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Please check if this issue is still valid on QGIS 3.4.5 or 3.6.

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• status_id was changed from Open to Feedback