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/view_content/attachments.tmpl

@@ -6,7 +6,7 @@
 		{{$hasThumbnails := false}}
 		{{- range .Attachments -}}
 			<div class="twelve wide column" style="padding: 6px;">
-				<a target="_blank" rel="noopener noreferrer" href="{{.DownloadURL}}" title='{{$.ctx.locale.Tr "repo.issues.attachment.open_tab" .Name}}'>
+				<a target="_blank" rel="noopener noreferrer" href="{{.DownloadURL}}" title='{{$.ctxData.locale.Tr "repo.issues.attachment.open_tab" .Name}}'>
 					{{if FilenameIsImage .Name}}
 						{{if not (containGeneric $.Content .UUID)}}
 							{{$hasThumbnails = true}}
@@ -31,7 +31,7 @@
 				{{if FilenameIsImage .Name}}
 					{{if not (containGeneric $.Content .UUID)}}
 					<a target="_blank" rel="noopener noreferrer" href="{{.DownloadURL}}">
-						<img src="{{.DownloadURL}}" title='{{$.ctx.locale.Tr "repo.issues.attachment.open_tab" .Name}}'>
+						<img src="{{.DownloadURL}}" title='{{$.ctxData.locale.Tr "repo.issues.attachment.open_tab" .Name}}'>
 					</a>
 					{{end}}
 				{{end}}