Refactor repo clone button and repo clone links, fix JS error on empty repo page (#19208)

The last PR about clone buttons introduced an JS error when visiting an empty repo page:
* https://github.com/go-gitea/gitea/pull/19028
* `Uncaught ReferenceError: isSSH is not defined`, because the variables are scoped and doesn't share between sub templates.

This:
1. Simplify `templates/repo/clone_buttons.tmpl` and make code clear
2. Move most JS code into `initRepoCloneLink`
3. Remove unused `CloneLink.Git`
4. Remove `ctx.Data["DisableSSH"] / ctx.Data["ExposeAnonSSH"] / ctx.Data["DisableHTTP"]`, and only set them when is is needed (eg: deploy keys / ssh keys)
5. Introduce `Data["CloneButton*"]` to provide data for clone buttons and links
6. Introduce `Data["RepoCloneLink"]` for the repo clone link (not the wiki)
7. Remove most `ctx.Data["PageIsWiki"]` because it has been set in the `/wiki` middleware
8. Remove incorrect `quickstart` class in `migrating.tmpl`

templates/base/head.tmpl

@@ -12,7 +12,7 @@
 	<meta name="keywords" content="{{MetaKeywords}}">
 	<meta name="referrer" content="no-referrer" />
 {{if .GoGetImport}}
-	<meta name="go-import" content="{{.GoGetImport}} git {{.CloneLink.HTTPS}}">
+	<meta name="go-import" content="{{.GoGetImport}} git {{.RepoCloneLink.HTTPS}}">
 	<meta name="go-source" content="{{.GoGetImport}} _ {{.GoDocDirectory}} {{.GoDocFile}}">
 {{end}}
 {{if .FeedURL}}