Add help text or dependency requirement to Service Accounts field in Admin Tool

[ldwright]

Background: if a user is not currently logged in to Sync we don't know which services they use and we default to "they don't use any service". On the other hand, if they are logged in, we can determine which services they have used or not.

Issue: Currently the Snippet Admin Tool allows you to select "No" for the Uses Firefox Sync field but still allows you to target "User has signed up for X" under the Service Accounts field.

• We should either clarify that this will not actually target anyone, or force the Uses Firefox Sync field to be "I don't care" or "Yes" if the Service Accounts drop-down is being utilized.