Don't use inline style

Ghost User
2019-06-03T17:22:10+02:00

thalia.nu uses an inline style attribute to set the header background image, this causes problems when setting the Content-Security-Policy (https://gitlab.science.ru.nl/thalia/serverconfig/merge_requests/380, https://gitlab.science.ru.nl/thalia/serverconfig/issues/248)

### Current behaviour
The background header image is set using an inline style attribute, CSP sends a [report](https://sentry.thalia.nu/sentry/content-security-policy-report/issues/103/events/3126/)

### Expected behaviour
Inline style attributes are not used on the thalia website

### Steps to reproduce

1. Go to thalia.nu
2. View CSP reports in network tab
3. Observe that reports are send about the inline style attribute for the background image