Fix view repo migration task logic

[wxiaoguang]

No description provided.

[wxiaoguang]

This is just an example about the XORM usage problem.

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

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. 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.