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 (https://github.com/qwerty287/gitea/blob/39eb82446c6fe5da3d79124e1f701f3795625b69/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/issue_subscription.go

@@ -279,9 +279,16 @@ func GetIssueSubscribers(ctx *context.APIContext) {
 		return
 	}
 	apiUsers := make([]*api.User, 0, len(users))
-	for i := range users {
-		apiUsers[i] = convert.ToUser(users[i], ctx.User)
+	for _, v := range users {
+		apiUsers = append(apiUsers, convert.ToUser(v, ctx.User))
 	}
 
+	count, err := models.CountIssueWatchers(issue.ID)
+	if err != nil {
+		ctx.Error(http.StatusInternalServerError, "CountIssueWatchers", err)
+		return
+	}
+
+	ctx.SetTotalCountHeader(count)
 	ctx.JSON(http.StatusOK, apiUsers)
 }