Attributes for email templates rendered conditionally, more added

This fixes #368 @ChrisBr @hennevogel please review
I'm not sure if this check is really necessary because it does not avoid that the variables are not replaced ... (e.g. if the admin adds these value to the registration, update date, update registration date or venue update mails). Furthermore the admin can only set up a proper email after at least one user has submitted a proposal, that means the first user doesn't get a mail! Not really what we want, I think!

A better approach would be, that we don't show these variables in the template help for registration, update date, update registration date or venue update mails. Something like this

```
= render partial: 'template_help', locals: { show_event_variables: true }
```