Fix Permission in API returned repository struct (#25388)

The old code generates `structs.Repository.Permissions` with only
`access.Permission.AccessMode`, however, it should check the units too,
or the value could be incorrect. For example,
`structs.Repository.Permissions.Push` could be false even the doer has
write access to code unit.

Should fix
https://github.com/renovatebot/renovate/issues/14059#issuecomment-1047961128
(Not reported by it, I just found it when I was looking into this bug)

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Review tips:

The major changes are
- `modules/structs/repo.go`
https://github.com/go-gitea/gitea/pull/25388/files#diff-870406f6857117f8b03611c43fca0ab9ed6d6e76a2d0069a7c1f17e8fa9092f7
- `services/convert/repository.go`
https://github.com/go-gitea/gitea/pull/25388/files#diff-7736f6d2ae894c9edb7729a80ab89aa183b888a26a811a0c1fdebd18726a7101

And other changes are passive.

modules/structs/repo.go

@@ -10,9 +10,9 @@ import (
 
 // Permission represents a set of permissions
 type Permission struct {
-	Admin bool `json:"admin"`
-	Push  bool `json:"push"`
-	Pull  bool `json:"pull"`
+	Admin bool `json:"admin"` // Admin indicates if the user is an administrator of the repository.
+	Push  bool `json:"push"`  // Push indicates if the user can push code to the repository.
+	Pull  bool `json:"pull"`  // Pull indicates if the user can pull code from the repository.
 }
 
 // InternalTracker represents settings for internal tracker