Add repr_html for CubeLists

[DPeterK]

As a (long-delayed) follow-on to a previous PR to add cube html repr (see #2918), this adds html repr functionality to CubeList objects. The html repr is designed to be reasonably simple, retaining the content of the original CubeList repr while adding an accordion effect that, when a cube in the list is clicked, expands to reveal the html repr of that cube.

[DPeterK]

For want of a properly interactive notebook that demonstrates this in operation, here's a static screengrab of what the repr looks like (with one cube in the list expanded to show the embedded cube repr):

[bjlittle]

@DPeterK Fancy doing a re-base?

We just fixed the travis-ci tests on master, and I'm hoping this will tidy up your CI failures.

Reference #3349

[bjlittle]

Ping @DPeterK 😉

[DPeterK]

@bjlittle ping noted; unfortunately I'm busy now! I'll see what I can do.

[bjlittle]

@DPeterK No worries 😉

[DPeterK]

@bjlittle I did it 😁

[DPeterK]

... shame the tests still didn't pass 😢

Edit: but it looks like it's just a license header thing 🎉 I'll fix...

[DPeterK]

I've updated the CSS for the cubelist repr, including making use of jupyter variables to define elements of the style. This means the cubelist repr will correctly follow jupyter lab themes. When combined with #3380 the cubelist repr now looks like the following:

Light theme:

Dark theme:

[bjlittle]

@DPeterK Awesome, LGTM 👍