Streamline UI and allow embedding in iframes

[philippotto]

tldr:

• allows to embed wk within an iframe
  • logging in does not work due to third-party cookies, but there will be a warning stating this
  • the golden layout config will not be persisted within an iframe and the default layout is switched to a two-column layout
• the wk-core "toolbar" was merged into the navbar to save vertical space and make the overall UI leaner
• some stylistic tweaks here and there

URL of deployed dev instance (used for testing):

• https://embeddablewk.webknossos.xyz

Steps to test:

• mainly play around with screen sizes and check basic functionality

Issues:

• fixes Optimize/trim down WK (within iframe) #3581
• fixes Having a small screen width causes the action bar to be rendered wrong #3891
• fixes Try to maximize area for rendering data #3521 imo
• fixes Tracing View: small design tweaks #3470

Screenshots

For small devices:

---

• Updated changelog
• [ ] Updated documentation if applicable (I created the follow-up issue Update Screenshots in Documentation #3912 for that)
• Ready for review

[philippotto]

@daniel-wer Maybe you can have a look at https://www.playframework.com/documentation/ja/2.3.x/SecurityHeaders to see whether we should keep some of these security measures?

[daniel-wer]

Incredible PR! I love how much thought you put into every single part of this, very well executed! 🏆
I can assure, opening up the "old" version of wK in a few weeks will be very strange (it already is for me, and I only reviewed this PR ^^), this is just so much better 👍

Things I noticed during testing:

• The spinner and percentage when saving large update actions no longer have a margin and are too close together.

• The Dasboard Icon does not show a title/tooltip when hovering.

• The highlighting of the visited sub page for the admin pages doesn't seem to work, "Dashboard" is always highlighted for me, regardless of whether I'm looking at statistics or task types pages, etc...

• The version restore view is a little too large at the top / the navbar is too tall when being logged out

• When logged out and looking at a tmpscratch dataset, the tmpscratch and anonymous user avatar are too close together
  

• What would be a good way to test the iframe-embeddability? Should we include a test .html page in the repo, which can simply be opened?

• I don't think this has been introduced in this PR, but some of the buttons in the action bar are rerendered in every frame when brushing in a volume tracing. I originally noticed this, because I thought brushing felt a little sluggish and then used the React Dev Tools - Highlight Updates to visualize. We should optimize this in another PR :)

[philippotto]

Thanks for your thorough feedback! I think the PR is good to go now. Will probably merge tomorrow morning.

[philippotto]

@daniel-wer If you give your official approval, I'd merge this :)