Fix HTML entities in the event organiser email

Ghost User
2018-06-13T19:04:40+02:00

Closes #625

#### Previous behaviour
Steps to reproduce:

1. Create an event
2. Add registration
3. Enable cancellation emails
4. Cancel registration

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

1. Create an event
2. Add regist...

Closes #625 (closed)

Previous behaviour

Steps to reproduce:

1. Create an event

2. Add registration

3. Enable cancellation emails

4. Cancel registration

New behaviour

Steps to validate that it works:

1. Create an event

2. Add registration

3. Enable cancellation emails

4. Cancel registration