Support new "Delete Reactions" endpoints

[wesleimp]

GitHub Developer API announcement:
https://developer.github.com/changes/2020-02-26-new-delete-reactions-endpoints/

These new endpoints need to be supported:

• Delete a commit comment reaction

  • DELETE /repos/:owner/:repo/comments/:comment_id/reactions/:reaction_id
  • Docs: https://developer.github.com/v3/reactions/#delete-a-commit-comment-reaction

• Delete an issue reaction

  • DELETE /repos/:owner/:repo/issues/:issue_number/reactions/:reaction_id
  • Docs: https://developer.github.com/v3/reactions/#delete-an-issue-reaction

• Delete a reaction to a commit comment

  • DELETE /repos/:owner/:repo/issues/comments/:comment_id/reactions/:reaction_id
  • Docs: https://developer.github.com/v3/reactions/#delete-an-issue-comment-reaction

• Delete a pull request comment reaction

  • DELETE /repos/:owner/:repo/pulls/comments/:comment_id/reactions/:reaction_id
  • Docs: https://developer.github.com/v3/reactions/#delete-a-pull-request-comment-reaction

• Delete team discussion reaction

  • DELETE /orgs/:org/teams/:team_slug/discussions/:discussion_number/reactions/:reaction_id
  • Docs: https://developer.github.com/v3/reactions/#delete-team-discussion-reaction

• Delete team discussion comment reaction

  • DELETE /orgs/:org/teams/:team_slug/discussions/:discussion_number/comments/:comment_number/reactions/:reaction_id
  • Docs: https://developer.github.com/v3/reactions/#delete-team-discussion-comment-reaction

[sreecodeslayer]

@edgurgel Hey man, How should we go about adding the teams-discussion (and then reactions) endpoints as I don't think they are currently supported.
CMIIW, but as per GitHub's API routes, the Discussions should be as : Tentacat.Organizations.Teams.Discussions namespace right?

Should I add the whole Discussions API as a single PR or do you want me to just add DELETE to close this PR? 😄