Export reaction users in JSON format

[slatinsky]

This pull request adds users that reacted to a message with an emoji to JSON export

"reactions": [
        {
          "emoji": {
            //...
          },
          "count": 37,
          "users": [
            {
              "id": "158600892784241132",
              "name": "user",
              "nickname": "user8745",
              "isBot": false,
              "avatarUrl": "..."
            },
            //...

• this change just adds a new users field - it doesn't break backwards compatibility
• user object in reactions is reduced - some fields are removed to improve export size (JsonMessageWriter.cs:67). This change doesn't affect users in other parts of export (backwards compatibility):
  • roles - for this we would need to request each user, too slow
  • color - requires roles
  • discriminator - not needed anymore
• only affecting JSON format - reaction users are fetched only if needed, so it doesn't affect other export formats other that json

notes:

• it affects only JSON format and doesn't break backwards compatibility. People exporting in JSON format expect this information to be included - I think that we don't need to add an option to turn this feature off

tested:

• pagination (> 100 users per emoji)
• windows 10 build + CLI export to JSON format

[slatinsky]

Changes:

• rebased with upstream
• readded discriminator, because some users are still not migrated
• refactored according to the code review

Tested on CLI build Windows 10, export to JSON format. Pagination (> 100 users per emoji) works too

[Tyrrrz]

Thanks, merged. I'll clean up in a follow up commit.