Refactor Pull Mirror and fix out-of-sync bugs (#24732)

The "mirror" table and "repository" table might be out-of-sync in some
cases.

It means that "IsMirror=true" but "Mirror=nil"

This PR removes unnecessary "Mirror" field, rename "Mirror" to
"PullMirror" and fix nil panic bug.


Screenshot of changed templates:


![image](https://github.com/go-gitea/gitea/assets/2114189/c0f2bdfc-5911-43ea-b989-b19619de4235)



![image](https://github.com/go-gitea/gitea/assets/2114189/1078b41a-484f-4c06-8c2f-edb9e658275d)

modules/context/repo.go

@@ -61,7 +61,6 @@ type Repository struct {
 	RepoLink     string
 	CloneLink    repo_model.CloneLink
 	CommitsCount int64
-	Mirror       *repo_model.Mirror
 
 	PullRequest *PullRequest
 }
@@ -380,13 +379,9 @@ func repoAssignment(ctx *Context, repo *repo_model.Repository) {
 	ctx.Data["Permission"] = &ctx.Repo.Permission
 
 	if repo.IsMirror {
-		ctx.Repo.Mirror, err = repo_model.GetMirrorByRepoID(ctx, repo.ID)
+		pullMirror, err := repo_model.GetMirrorByRepoID(ctx, repo.ID)
 		if err == nil {
-			ctx.Repo.Mirror.Repo = repo
-			ctx.Data["IsPullMirror"] = true
-			ctx.Data["MirrorEnablePrune"] = ctx.Repo.Mirror.EnablePrune
-			ctx.Data["MirrorInterval"] = ctx.Repo.Mirror.Interval
-			ctx.Data["Mirror"] = ctx.Repo.Mirror
+			ctx.Data["PullMirror"] = pullMirror
 		} else if err != repo_model.ErrMirrorNotExist {
 			ctx.ServerError("GetMirrorByRepoID", err)
 			return