MDL-48493 admin: Move the acknowledgement to the plugin validation page

Previously, the acknowledgement was required only when installing
plugins from the ZIP file. It is valid when installing plugins from the
Plugins directory, too. So we display it at the validator page now to
make sure it is displayed in both cases.

There is no need to require the admin to check the acknowledgement box
now. We do not do that anywhere in admin UI (and there are more
dangerous operations than installing a plugin). Admins still can deploy
plugins manually without this tool anyway. And at the end, they are
admins. They should know what they are doing. Simply displaying the
acknowledgement message before they click the install button is enough.