[enh] add --purge option in app uninstall modal

[Axolotle]

Display a checkbox to purge the datadir if the app has support_purge: true
Should fix YunoHost/issues#1854

Also fix good_practices_about_admin_password not being translated in forms.

[alexAubin]

(Status update : we need to add the definition of support_purge in the core ... maybe with appv2 + check that there's a data_dir in the app resources)

[Axolotle]

superseded by #532