Fix 0013085: Allow Manage Tags only for global level >= tag_edit_threshold

Previously the level check in function print_manage_menu was made at project
level, which is not consistent with globally managed Tags.

The access level check change in manage_tags_page.php is for consistency,
only to avoid users without the proper access level trying to reach the page
directly, even though it would only display a read-only list of tags.

Signed-off-by: David Hicks d@hx.id.au