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`

web_src/js/features/repo-legacy.js

@@ -16,7 +16,7 @@ import {svg} from '../svg.js';
 import {htmlEscape} from 'escape-goat';
 import {initRepoBranchTagDropdown} from '../components/RepoBranchTagDropdown.js';
 import {
-  initRepoClone,
+  initRepoCloneLink,
   initRepoCommonBranchOrTagDropdown,
   initRepoCommonFilterSearchDropdown,
   initRepoCommonLanguageStats,
@@ -498,7 +498,7 @@ export function initRepository() {
     initRepoCommonFilterSearchDropdown('.choose.branch .dropdown');
   }
 
-  initRepoClone();
+  initRepoCloneLink();
   initRepoCommonLanguageStats();
   initRepoSettingBranches();