Lazy render emoji dropdown

What does this MR do and why?

fix #498419 (closed) - performance issue and lots of network requests for custom emojis.

Removes potentially lots of DOM nodes, and prevents lots of duplicate network requests.

How to test

1. Add some custom emojis

   mutation CreateCustomEmoji($groupPath: ID!) {
     createCustomEmoji(input: {groupPath: $groupPath, name: "party-parrot", url: "https://cultofthepartyparrot.com/parrots/hd/parrot.gif"}) {
       clientMutationId
       customEmoji {
         name
       }
       errors
     }
   }

2. Load an issue with some comments on it
3. Check network responses - should be fewer than without this change
4. Check DOM (in Elements tab, search for "Search for an emoji" with cmd+f searchbar) - there should only be a picker when it has been opened