[Feature Request] Webhooks message customization

[davydov-vyacheslav]

I'd like Gitea has customized messages for Webhooks.

As for now, Gitea Webhooks messages are hardcoded and there is no way for customization.

I think that following things make Gitea Webhooks functionality much better:

• on webhooks page (maybe even better in config file) add Custom message field where user is able to set own message for specific action
• user should be allowed to have/use predefined "variables", like %repo% for link to org/repository with the name,%issue.title% - issue's title if the action type in scope of issue and so on.

And as result, I'd like to be able to set (e.g. for pull request) message like: %user% has submitted pull-request to %repo%.

[jolheiser]

This would be covered by #1089