Bump panel to 1.4.0

[pmeier]

Was released today: https://blog.holoviz.org/posts/panel_release_1.4/ There are a few chat improvements that we should take a look at: https://blog.holoviz.org/posts/panel_release_1.4/#chat-improvements.

[pmeier]

Let's do a two-step approach here:

1. Upgrade our requirement to panel==1.4.0 just making sure that everything that worked before still works.
2. Check out the improvements that came with the new release and see if we can benefit from that.

This split is useful, since #312 can already be merged after 1. and I don't want to delay it any further.

[pmeier]

Testing locally, I noticed two differences:

Loading indicator

loading_indicator_1_3_8.mp4

loading_indicator_1_4_0.mp4

Accordion rendering

Since both things are purely styling, I'm also ok to just merge the bump to 1.4.0 and fix the styling in #312. @pierrotsmnrd Thoughts?