Revert "Don't use Gtk.InfoBar yet as it requires pyGtk >= 2.22. We'll keep this in a branch for now."

This reverts commit cb1e224cdebd8dce9569621f050f4ef8203254e5.
I18n offer_url parameters.
In offer_url(), use InfoBar instead of MessageDialog.
When exporting from recipe card, display InfoBar there.
Replace Jump to button in successful export infobar by link to file.
Move import/export progress bar to InfoBar.
Use InfoBar for message in duplicate finder after website import.