can't add emoji to issues

- Gitea version (or commit ref): 1.6.3
- Database (use `[x]`):
  - [x] MySQL

## Description
Can't add emoji to issues. I get this error:

`[...outers/repo/issue.go:1192 NewComment()] [E] CreateIssueComment: CreateComment: Error 1366: Incorrect string value: '\xF0\x9F\xA5\xB6' for column 'content' at row 1`
