Refactor "htmx" to "fetch action" (#37208)

The only remaining (hard) part is "templates/repo/editor/edit.tmpl", see the FIXME

By the way:

* Make "user unfollow" use basic color but not red color, indeed it is not dangerous
* Fix "org folllow" layout (use block gap instead of inline gap)

services/gitdiff/gitdiff.go

@@ -228,7 +228,7 @@ func (d *DiffLine) RenderBlobExcerptButtons(fileNameHash string, data *DiffBlobE
 			link += fmt.Sprintf("&pull_issue_index=%d", data.PullIssueIndex)
 		}
 		return htmlutil.HTMLFormat(
-			`<button class="code-expander-button" hx-target="closest tr" hx-get="%s" data-hidden-comment-ids=",%s,">%s</button>`,
+			`<button class="code-expander-button" data-fetch-sync="$closest(tr)" data-fetch-url="%s" data-hidden-comment-ids=",%s,">%s</button>`,
 			link, dataHiddenCommentIDs, svg.RenderHTML(svgName),
 		)
 	}