Bugfix: Bulk actions widget overlaid course header image, solves #469.

[abias]

Before:

After:

This was a quick fix in CSS, but it should be sufficient from my point of view.

While the "bulk actions" widget on Moodle 4.3 is the first widget where I realized this header actions area myself, the header actions area has been there before on Moodle 4 already and might be filled by course formats or plugins. Thus, it should be safe to backport this patch to pre-4.3 versions of Boost Union as well.

[lucaboesch]

I see what you are doing there, and I am willing to approve this (in fact I do) but the more proper way would be to add dynamically add a .button-secondary class to it or load button-secondary SCSS mixins, I think.

[abias]

Thank you for your approval, Luca.
I understand your concerns. However, I think that we cannot rely on the fact that a button is displayed in this header actions area. I looked a little bit closer and saw that it just needs a all to $PAGE->add_header_action() to show content in this area.

That's why I just made sure that the content, whateever it is, is displayed on a white background just as Boost Core does it.

I will now rebase the branch and merge it as you have approved it.

Cheers