* Add Issue webhook payload
* Add copyright header
* Whoops, wrote license header in wrong file...
* Move fields of PR/Issue payload to BaseIssuePayload
* Fix license header
* remove BaseIssuePayload
* Add comment to IssuePayload.JSONPayload
* Fix comment mentioning IssuePayload
* Reorder structs
* Mimic GitHub webhooks
Since the Gitea webhooks currently mimic for a big part the way GitHub does webhook, issue comments HookIssueActions have been removed to be added again at a later stage and milestone_set and milestone_cleared have been changed to milestoned and demilestoned, respectively.
* Change also constant names
