Refactor ctx in templates (#23105)

Before, the `dict "ctx" ...` map is used to pass data between templates.

Now, more and more templates need to use real Go context:

* #22962
* #23092


`ctx` is a Go concept for `Context`, misusing it may cause problems, and
it makes it difficult to review or refactor.

This PR contains 2 major changes:

* In the top scope of a template, the `$` is the same as the `.`, so the
old labels_sidebar's `root` is the `ctx`. So this `ctx` could just be
removed.
bd7f218dce
* Rename all other `ctx` to `ctxData`, and it perfectly matches how it
comes from backend: `"ctxData": ctx.Data`.
7c01260e1d



From now on, there is no `ctx` in templates. There are only:

* `ctxData` for passing data
* `Context` for Go context

templates/repo/issue/new_form.tmpl

@@ -80,7 +80,7 @@
 					{{end}}
 				</div>
 			</div>
-			{{template "repo/issue/labels/labels_sidebar" dict "root" $ "ctx" .}}
+			{{template "repo/issue/labels/labels_sidebar" dict "root" $}}
 
 			<div class="ui divider"></div>