Reduced motion sickness animations

In many places of Nextcloud, motion sickness is induced, for example:

• loading icons
• modal loadings (they grow a bit when opened)
• app navigation bar moving upwards when hovered
• Scrolling
• and more I may have not seen yet

Solution:
There is a setting for “reduced motion animations available on every operating system which should be respected by the library.
This website explains how to do it and where to find the settings on each OS: https://developer.mozilla.org/en-US/docs/Web/CSS/@media/prefers-reduced-motion

As an example, I made this change for custom animations in Assistant in this PR: https://github.com/nextcloud/assistant/pull/365

Not all animations are inducing motion sickness, only things that move parts of the website in relation to other parts, or animations that “move” a color gradient. Elements appearing or fading are fine in that regard and can still be used, also as a replacement if the setting is activated. If the movement is supposed to trasmit essential information to the user, it should not be just removed but be replaced by something else transferring that information (which can also be a color, an icon, …)

No files or tests are named. Start by inventorying the library's motion-producing animations and review the linked MDN guidance and Assistant example; done means motion-sensitive animations respect prefers-reduced-motion while essential information remains available through another cue.