Keep pannel close button accessible with external styles

[TomHall2020]

Description

External styles such as missing.css can interfere with the display of the panel header elements, and since these are currently rendered after the button they can appear ontop and block click events from passing through. Fix was fairly straightforward, have just swapped the order of display everwhere I could find these pairs of elements.

Fixes #2083

While the selenium tests are all working, I haven't quite figured out how I could incorporate a regression test for this. I've narrowed the offending css down to this rule:

h1,h2,h3,h4,h5,h6 {
  position:relative;
} 

And I've tested manually that this will break the close button before this PR and not after, but I can't quite see how to approach setting up a test case for this. The test cases I've edited do at least assert that the content of the button is rendered after the headers, so it is indirectly tested for, perhaps that is suffient?

Checklist:

• I have added the relevant tests for this change (edited existing tests)
• I have added an item to the Pending section of docs/changes.rst.

[matthiask]

Thank you!