Gitea contribution heatmap private activity disclosure to public-only tokens

`GET /api/v1/users/{username}/heatmap` computed the contribution heatmap with the repository access of the token owner and ignored the public-only restriction of the access token, unlike the activity feed endpoints. A holder of a user's public-only token could read contribution counts and their 15-minute time buckets that stem from that user's activity in private repositories. Repository names and contents were not disclosed. For public-only tokens the heatmap now only counts activity visible to anonymous users.