Webhook empty text

[FriedrichVonSchnitzel]

• Gitea version (or commit ref): a757920
• Git version:
• Operating system: Debian (docker)
• Database (use [x]):
  • PostgreSQL
  • MySQL
  • MSSQL
  • SQLite
• Can you reproduce the bug at https://try.gitea.io:
  • Yes (provide example URL)
  • No
  • Not relevant
• Log gist:

Request
Response
200
Headers
Request URL: https://hooks.slack.com/services/.../.../..........
Request method: POST
Content-Type: application/json
X-GitHub-Delivery: 4ebaceaf-ecfd-4522-8873-bdff299d4244
X-GitHub-Event: issues
X-Gitea-Delivery: 4ebaceaf-ecfd-4522-8873-bdff299d4244
X-Gitea-Event: issues
X-Gogs-Delivery: 4ebaceaf-ecfd-4522-8873-bdff299d4244
X-Gogs-Event: issues
Content
{
  "channel": "#channel",
  "text": "",
  "username": "Gitea",
  "icon_url": "http://domain.com:3000/img/gitea-sm.png",
  "unfurl_links": 0,
  "link_names": 0,
  "attachments": [
    {
      "fallback": "",
      "color": "",
      "title": "",
      "text": ""
    }
  ]
}

Description

Webhooks to slack contain empty texts when notifying about milestones assigned to issues.
The bug occurs regardless of what is selected in the webhook settings (Trigger on: All events / Custom Events... - all selected). Example Webhook request from "Recent deliveries" provided above.
...

Screenshots

[KubqoA]

It's probably due to the fact, that the slack webhook implementation for issues doesn't check for api.HookIssueMilestoned and api.HookIssueDemilestoned. Maybe I could write you a PR.