Improve positioning of titles on pages with a limited width

[chaoran-chen]

@theosanderson wrote in #1564 (comment)

I think on pages where we limit the width - e.g. the submit form, and the portal - we need to limit the width that the header can take up too. But we can do that in a future PR if you prefer. I know it was a pre-existing issue (slightly exacerbated by now having the group selector there).

Example screenshot: