Fix javascript error when an anonymous user visiting migration page

[lunny]

This PR fixes javascript errors when an anonymous user visits the migration page.
It also makes task view checking more restrictive.

The router moved from /user/task/{id}/status to /username/reponame/-/migrate/status because it's a migrate status.

[yp05327]

How to reproduce?

[lunny]

How to reproduce?

migrate a big repository and visit the repository with anonymous.

[wxiaoguang]

JS: foo()?.bar()

[lunny]

JS: foo()?.bar()

e0a9e25

dismiss

[wxiaoguang]

-> Fix view repo migration task logic #32152

Ideally it should be like this.

Update: 32152 is just an example about the XORM usage problem.

---

Actually, the "repo migrating" page has a lot of problems.

This PR is far from ideal either (actually I wouldn't say this PR is right).

1. The "repo migrating" page is shown under /{owner}/{repo} routing path, but it uses /user/task/{id} to query the task information. It breaks the repo's permission model.
2. The permission check is quite messy. For example: /user/task/{id} checks doer_id, but on the page it checks IsAdmin (what if another admin would like to manage the task?). Ideally it shouldn't check doer ID, but just respect the repo's permission.

To make things right, it needs to use something like /{owner}/{repo}/-/migrating-task to query the current repo's task state, and allow "admin" roles to delete or retry.

[GiteaBot]

I was unable to create a backport for 1.22. @lunny, please send one manually. 🍵

go run ./contrib/backport 32144
...  // fix git conflicts if any
go run ./contrib/backport --continue