Make plugin license required as of 03 June

[Xpirix]

This is for #38 according to https://github.com/qgis/QGIS-Enhancement-Proposals/issues/279

Changes summary

• Make the plugin license file required for any new updates to existing and new plugins published
• Update unit test

Please find below the screenshots of the error raised when a license file is missing:

For a new plugin:

For a new version of an existing plugin: