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`

routers/web/web.go

@@ -984,6 +984,7 @@ func RegisterRoutes(m *web.Route) {
 			m.Get("/commit/{sha:[a-f0-9]{7,40}}.{ext:patch|diff}", repo.RawDiff)
 		}, repo.MustEnableWiki, func(ctx *context.Context) {
 			ctx.Data["PageIsWiki"] = true
+			ctx.Data["CloneButtonOriginLink"] = ctx.Repo.Repository.WikiCloneLink()
 		})
 
 		m.Group("/wiki", func() {