introduce the browseheader_message macro
    
    Introduces a new macro for themes, the browseheader_message.
    This allows a theme to define the message that is shown on the
    browse pages. In the default theme, this message was and is only
    shown on the browse projects.
    
    This also updates the srcfpo theme to use this new macro, and remove
    the overridden template that was being used to do this.
    
    Fixes #3617