Add missing X-Total-Count and fix some related bugs (#17968)

* Add missing `X-Total-Count` and fix some related bugs

Adds `X-Total-Count` header to APIs that return a list but doesn't have it yet.
Fixed bugs:
* not returned after reporting error (39eb82446c/routers/api/v1/user/star.go (L70))
* crash with index out of bounds, API issue/issueSubscriptions

I also found various endpoints that return lists but do not apply/support pagination yet:
```
/repos/{owner}/{repo}/issues/{index}/labels
/repos/{owner}/{repo}/issues/comments/{id}/reactions
/repos/{owner}/{repo}/branch_protections
/repos/{owner}/{repo}/contents
/repos/{owner}/{repo}/hooks/git
/repos/{owner}/{repo}/issue_templates
/repos/{owner}/{repo}/releases/{id}/assets
/repos/{owner}/{repo}/reviewers
/repos/{owner}/{repo}/teams
/user/emails
/users/{username}/heatmap
```
If this is not expected, an new issue should be opened.

Closes #13043

* fmt

* Update routers/api/v1/repo/issue_subscription.go

Co-authored-by: KN4CK3R <admin@oldschoolhack.me>

* Use FindAndCount

Co-authored-by: KN4CK3R <admin@oldschoolhack.me>
Co-authored-by: 6543 <6543@obermui.de>

routers/api/v1/repo/tree.go

@@ -63,6 +63,7 @@ func GetTree(ctx *context.APIContext) {
 	if tree, err := files_service.GetTreeBySHA(ctx.Repo.Repository, sha, ctx.FormInt("page"), ctx.FormInt("per_page"), ctx.FormBool("recursive")); err != nil {
 		ctx.Error(http.StatusBadRequest, "", err.Error())
 	} else {
+		ctx.SetTotalCountHeader(int64(tree.TotalCount))
 		ctx.JSON(http.StatusOK, tree)
 	}
 }