Add warning when no conditions and/or actions are available

Simplest thing is to implement hook_store_status.
A warning when users actually are on the page with the empty bits, or when users are at the start of the process of adding a discount would be good too -- since there's no point creating a discount if you have less than 1 condition and 1 action available.

/admin/store/discounts ?

Yup, since that's where you start trying to add discounts.
Still, it's not too hard to also do a status notice.

So we need to code:
- a sanity-checking function for the core discounts module. This checks there is at least one action and one condition available; returns TRUE or FALSE.
- something that calls this on the discounts admin page. Ideally, this should block the 'add discount' link too, since it's of no use.
- an implementation of hook_store_status which calls this too.

Turns out there's already functions that get a list of actions and conditions: they're used to populate those lists that are showing up empty.
Implemented the hook, and checking on our admin page too with a message that sends the user to check status.
Here's a patch of what I've committed.