Add /gif to the list of slash commands

GIFs are one of the most expressive ways to communicate online in my opinion. We should try to integrate them into our comments.

The approach I have in mind would be to leverage our slash commands to do something like /gif GIF_NAME which we would then parse and append the right GIF into the comment just as Slack does.

Thoughts @DouweM @JobV @smcgivern ?

I am willing to work on this feature as on my spare time as a community contribution 😄