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)

templates/repo/editor/edit.tmpl

@@ -23,6 +23,8 @@
 										<a class="active item" data-tab="write">{{svg "octicon-code"}} {{if .IsNewFile}}{{ctx.Locale.Tr "repo.editor.new_file"}}{{else}}{{ctx.Locale.Tr "repo.editor.edit_file"}}{{end}}</a>
 										<a class="item{{if not .CodeEditorConfig.Previewable}} tw-hidden{{end}}" data-tab="preview" data-preview-url="{{.Repository.Link}}/markup" data-preview-context-ref="{{.RepoLink}}/src/{{.RefTypeNameSubURL}}">{{svg "octicon-eye"}} {{ctx.Locale.Tr "preview"}}</a>
 										{{if not .IsNewFile}}
+										{{/*FIXME: the related logic is totally a mess, need to completely rewrite, that's also the root reason for
+												why the "migrate to CodeMirror" PR  took very long time on the legacy code and introduced "#file-name (filenameInput)" regressions many times*/}}
 										<a class="item" data-tab="diff" hx-params="context,content" hx-vals='{"context":"{{.BranchLink}}"}' hx-include="#edit_area" hx-swap="innerHTML" hx-target=".tab[data-tab='diff']" hx-indicator=".tab[data-tab='diff']" hx-post="{{.RepoLink}}/_preview/{{.BranchName | PathEscapeSegments}}/{{.TreePath | PathEscapeSegments}}">{{svg "octicon-diff"}} {{ctx.Locale.Tr "repo.editor.preview_changes"}}</a>
 										{{end}}
 									</div>