Rebrand loading page for Endless Key

[GeorgesStavracas]

See first and second commit. The last commit is merely a reproduction of what translation changes would look like.

I'd like to get some feedback here first, before applying this change to all files.

A nice follow-up improvement where we inject a loading page might be feasible since learningequality/kolibri-app#110 has been merged. I haven't looked at it yet.

[GeorgesStavracas]

This is how it looks right now:

Gravacao.de.tela.de.2023-09-12.15-34-13.webm

The error screen:

(I did not change error strings!)

[dylanmccall]

That seems fine for the moment, at least so we have a nice consistent-looking experience to start with. I think it would be okay to drop the localized versions for now so we don't have to deal with those. We can add a follow-up task to switch to something like what learningequality/kolibri-app is doing now, or the same thing we're doing in endlessm/kolibri-installer-android (but it would be nice if that was also doing the shiny new thing from kolibri-app).

[dbnicholson]

I think this all goes away after #72 and any further improvements should be made in the loading-screen package in kolibri-explore-plugin.

[dylanmccall]

Yep, we can close this. With that said, I kind of want to get the Explore plugin to do a normal template override for the loading screen, and this branch could be a useful reference for that.