MDL-39087 Clarify plugininfo_base::get_uninstall_url() return value
The method now returns null if there should be no 'Uninstall' link at
the Plugins management screen. For non-standard add-ons the method now
returns URL to a general uninstall tool.
Plugin info subclasses can still override the method to provide URL to
their own UI for uninstalling. If the plugin type wants to use the
general uninstall tool also for standard plugins, it should override
this method and explicitly return $this->get_default_uninstall_url().
Otherwise, the 'Uninstall' link will be provided for add-ons only.
