Do not allow page_head() to be called twice

[Rytiss]

Description of the Change

error_page() calls page_head(), but there are instances of code that call error_page() having already called page_head() before, thus attempting to send headers twice and creating double page header.

Fixes #2960

Release Notes
Prevent page_head() from being called multiple times. Notice to projects: If you have a page_head() function override in your project.inc you need to apply this change there too.

[ChristianBeer]

This definitely fixes the problem but there should be an entry in the server release notes because existing projects that have this function overridden in their project config need to also change those or still suffer from the problem.

[Rytiss]

This definitely fixes the problem but there should be an entry in the server release notes because existing projects that have this function overridden in their project config need to also change those or still suffer from the problem.

Updated the release notes field in the original description above. Do I need to do anything else?

[ChristianBeer]

I added: "Notice to projects: If you have a page_head() function override in your project.inc you need to apply this change there too." to the release note and confirmed that it fixes the issue.

Thanks for your contribution.