Make the entire image top bar sticky

[ghuds540]

5 days ago #6320 was merged, which implements a sticky behavior for the image selection toolbar. However, the regular image top bar itself is not sticky. This behavior is in contrast to the top bar of the Scene tab, which is sticky.

This PR makes the entire Image top bar sticky (and adjusts some styles to make it clean), which matches the behavior of the top bar in Scene search.

scenes top-bar (vanilla):

image top-bar (vanilla):

image top-bar sticky (this PR):

Another side-effect of this change was that it caused the Scenes top-bar to move to the center, but I think it looks a lot better:

scenes top-bar (vanilla):

scenes top-bar (this PR):

I ran make validate and CSS linting passed.

[ghuds540]

re: this PR causing the scenes top bar to move to the center

I'm not sure what y'all have in mind for the new Scene filter and side panel, so let me know if I should make this CSS change a more targeted fix for the image bar container specifically. Thanks!