Return RFC3339 UTC timestamps for machine-readable output (#470)

### ⚠️ breaking changes ⚠️

- unset timestamps will not be printed as `"0001-01-01 00:00"`, but as empty value `""`
- output formats `csv`, `tsv`, `yaml` output timestamps in UTC instead of local time, and adhere to [RFC3339](https://datatracker.ietf.org/doc/html/rfc3339)

Co-authored-by: Norwin <git@nroo.de>
Co-authored-by: 6543 <6543@obermui.de>
Reviewed-on: https://gitea.com/gitea/tea/pulls/470
Reviewed-by: 6543 <6543@obermui.de>
Reviewed-by: John Olheiser <john.olheiser@gmail.com>
Co-authored-by: Norwin <noerw@noreply.gitea.io>
Co-committed-by: Norwin <noerw@noreply.gitea.io>

modules/print/comment.go

@@ -38,12 +38,12 @@ func Comment(c *gitea.Comment) {
 func formatComment(c *gitea.Comment) string {
 	edited := ""
 	if c.Updated.After(c.Created) {
-		edited = fmt.Sprintf(" *(edited on %s)*", FormatTime(c.Updated))
+		edited = fmt.Sprintf(" *(edited on %s)*", FormatTime(c.Updated, false))
 	}
 	return fmt.Sprintf(
 		"---\n\n**@%s** wrote on %s%s:\n\n%s\n",
 		c.Poster.UserName,
-		FormatTime(c.Created),
+		FormatTime(c.Created, false),
 		edited,
 		c.Body,
 	)