Move issue pin to an standalone table for querying performance (#33452)

Noticed a SQL in gitea.com has a bigger load. It seems both `is_pull`
and `pin_order` are not indexed columns in the database.

```SQL
SELECT `id`, `repo_id`, `index`, `poster_id`, `original_author`, `original_author_id`, `name`, `content`, `content_version`, `milestone_id`, `priority`, `is_closed`, `is_pull`, `num_comments`, `ref`, `pin_order`, `deadline_unix`, `created_unix`, `updated_unix`, `closed_unix`, `is_locked`, `time_estimate` FROM `issue` WHERE (repo_id =?) AND (is_pull = 0) AND (pin_order > 0) ORDER BY pin_order
```

I came across a comment
https://github.com/go-gitea/gitea/pull/24406#issuecomment-1527747296
from @delvh , which presents a more reasonable approach. Based on this,
this PR will migrate all issue and pull request pin data from the
`issue` table to the `issue_pin` table. This change benefits larger
Gitea instances by improving scalability and performance.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

models/fixtures/issue_pin.yml

@@ -0,0 +1,6 @@
+-
+  id: 1
+  repo_id: 2
+  issue_id: 4
+  is_pull: false
+  pin_order: 1