Count only visible notes

Fix counting the number of notes to skip those that are not visible
according to the user's access levels (private notes).

Bugnote count is built alongside bugnote statistics.

This commit rewrites the statistics generation to account for user
access level for each bug and each project.

Removes duplicate logic (sql queries) from:

• bug_api.php: function bug_get_bugnote_stats()
• filter_api.php: function filter_get_bug_rows()

Fixes: 0006939