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/repo/migrate/migrating.tmpl

@@ -1,5 +1,5 @@
 {{template "base/head" .}}
-<div class="page-content repository quickstart">
+<div class="page-content repository">
 	{{template "repo/header" .}}
 	<div class="ui container">
 		<div class="ui grid">