Allow custom award emoji through the API

What does this MR do and why?

This change allows users to award custom emoji to issues/MRs/comments.

Screenshots or screen recordings

How to set up and validate locally

1. Enable the custom_emoji feature flag

2. Make sure you've added custom emoji.

   • You can do this through the GraphQL API:
     1. Browse to /-/graphql-explorer
     2. And type and run:

     mutation {
       createCustomEmoji(input: { groupPath: "gitlab-org", name: "troll", url: "https://upload.wikimedia.org/wikipedia/en/9/9a/Trollface_non-free.png"}) {
         clientMutationId
         errors
         customEmoji {
           name
         }
       }
     }

   • Or on the Rails console:

     CustomEmoji.create(namespace: Group.first, name: 'troll', file: 'https://upload.wikimedia.org/wikipedia/en/9/9a/Trollface_non-free.png', external: true, creator: User.first)

3. Visit any issue. And get the API URL for that issue, being <group>%2f<project>/issues/<iid>

4. Award a custom emoji using curl:

   curl -L --request POST --header "PRIVATE-TOKEN: $GITLAB_TOKEN" "http://gdk.test:8080/api/v4/projects/flightjs%2fFlight/issues/6/award_emoji?name=troll"

5. Visit the issue again, and see the awarded emoji.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.