[Self-hosting] Login Slideshow Background

Since i've seen fluxer add the backgrounds to instances, thought of something that could be in a future update depending on how it goes...

Adding transition/slideshow backgrounds to fluxer similar to the OG Discord back in 2015-2017 how it was a different background sometimes when you logged in, that it had one background and another at separate changes. Alongside making it a choice whether selfhosting owners/admin want it to have multiple backgrounds or just one.

Allow users to choose more than one background, and the option to change it.