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>

models/issue_comment.go

@@ -593,7 +593,7 @@ func (c *Comment) loadReactions(e db.Engine, repo *repo_model.Repository) (err e
 	if c.Reactions != nil {
 		return nil
 	}
-	c.Reactions, err = findReactions(e, FindReactionsOptions{
+	c.Reactions, _, err = findReactions(e, FindReactionsOptions{
 		IssueID:   c.IssueID,
 		CommentID: c.ID,
 	})