adds nonce and role verification to entry exports.

[avonville]

Fixes weforms pro issue-139

Testing

Nonce Testing

1. Click on All forms
2. Click on View Entries under a form
3. Right click and copy link address for Export Entries
4. Paste that link in a new tab and remove nonce at the end i.e. &_wpnonce=02a2466c09
5. Should get an invalid nonce message.

6. Role Testing

7. Create a new user with a role less than admin.
8. Click on All forms
9. Click on View Entries under a form
10. Right click and copy link address for Export Entries
11. Paste that link in a new window where the new user is logged in.
12. Should get an invalid permission message.