Make the separator between apps in Appstore visible

[danielhollas]

In the appstore we render all apps below each other. There is <hr> element as a visual separator, but it is not that much visible, (on one of my monitors with lower brightness, it is actually invisible).

aiidalab-home/home/app_store.py

Line 170 in 0b1c618

display(ipw.HTML("<hr>")) # horizontal line

, but (at least in my browser) it is not visible since its color is white.

We need to add some styling to it. We should also adjust the overall whitespace around the apps, it seems currently there's a bit too much of it.

[danielhollas]

Styling an <hr> tag seems to be bit of a pain, see

https://developer.mozilla.org/en-US/docs/Web/HTML/Element/hr
https://www.w3schools.com/tags/tryit.asp?filename=tryhtml_hr_align_css
https://stackoverflow.com/questions/4151743/how-can-i-change-the-thickness-of-my-hr-tag
https://www.thoughtco.com/styling-horizontal-rule-tag-3466399

It would be nicer instead of inserting a separate <hr> in between tags to utilize the ::before CSS pseudoclass. Alternatively, we could use a top border around the app.

[danielhollas]

As part of this, we could also take a look at removing supposedly superfluouos HBoxes, see #130 (comment)