Base costs warning in newsletter only on the checkbox

Ghost User
2019-04-24T18:54:24+02:00

Closes #860

#### Previous behaviour
Steps to reproduce:

1. Create a newsletter with an event
2. The show warning if costs is checked
3. The price is 0
4. No warning shown

#### New behaviour
Steps to validate that it works:

1. Create ...

Closes #860 (closed)

Previous behaviour

Steps to reproduce:

1. Create a newsletter with an event

2. The show warning if costs is checked

3. The price is 0

4. No warning shown

New behaviour

Steps to validate that it works:

1. Create a newsletter with an event

2. The show warning if costs is checked

3. The price is >0

4. Warning shown