Add inline avatar in reactions tooltip

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Is your feature request related to a problem? Please describe.
When hovering on a reaction button, one can see the list of people who reacted in plain text separated by commas.
This doesn't feel very confortable, especially when there's a longer list.

Describe the solution you'd like
Add mini avatars in that list in front of the names as people are trained already to recognize avatars which makes it quicker to read.

Describe alternatives you've considered
Instead of mini avatars, perhaps use something like the mention bubbles, but I'm not sure if this will look good.

Additional context
None. Reactions are awesome, thanks for having added them 😄

Start by locating the reaction button tooltip in the JavaScript frontend and inspect how the current comma-separated names are rendered. The feature is done when the tooltip shows a small avatar before each reacting person's name while retaining the list of people.