Fix some small bugs in the automatic newsletter event code

Closes #861 (closed)

Previous behaviour

Steps to reproduce:

1. Create a newsletter event and automatic dropdown
2. Entered end date does not match the end date of the event

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1. The costs are never shown in the newsletter preview

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1. The automatic event selection always uses the dutch event title in the dropdown

New behaviour

Steps to validate that it works:

1. Create a newsletter event and automatic dropdown
2. Entered end date matches the end date of the event

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1. The costs are shown in the newsletter preview when price > 0 and the checkbox is checked

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1. The automatic event selection uses the current language title in the dropdown