Refactor reaction handler Typescript

[BurntimeX]

Remaining tasks:

• .reactionButton actually needs an aria-owns attribute
• According to the ARIA Guide, buttons in the popover should have tabindex=-1 (but then focus-trap stops working — is it even necessary anymore? Is this configurable?)
• Hide on window resize (is there another way to handle this?)
• Support for scrollable popovers (too many reaction types) is currently missing (maybe there’s another way to solve this?)