Investigate more usages of transitions and animations to remove for users with `prefer-reduced-motion`

[canova]

We've removed some animations in #3972. But we do have some more transitions as Julien wrote in his review message. We should investigate them and remove them for users with prefer-reduced-motion if they are a problem for them.

Here's the list:

• the SVG icon when uploading res/img/svg/sharing-animated-dark-12.svg. But maybe that's OK because it's fairly small.
• in the menu buttons, when the panel is dismissed
• the component showing up when doing drag and drop from a file
• The transition when reordering tracks
• The introduction on the search input
• Transitions in the active tab view, when opening tracks
• Possibly the transition when the "+" button appears when doing a selection
• The help about scrolling in Viewport