Eliminate automatic bulk permission syncing (i.e. ditch view.accessconf)

It's confusing that we make secret URLs and sharing into two separate screens. Many other web applications also offer the functionality to create a secret URL, and they group it under the other sharing permissions.

I believe the reason we have to separate them is because of how we handle bulk permissions currently. That is, every page & collection in Mahara has a "accessconf" field in the database, which is a hash of all their permissions. When you go to the Sharing screen, we look or other pages & collections with an identical hash (which means they have identical permissions), and we pre-tick the checkboxes for those ones.

In essence, this means that every visit to the "Sharing" screen is treated as a "bulk" visit. And since secret URLs apply to an individual Page/Collection and not in bulk, it forces us to make them separate.

But this is not a great way to do this. It often "gotchas" our users. For instance, if you go to edit the permissions on a page you've made Public, you will automatically edit the permissions for all of your Public pages, and you probably won't realize that, especially if you have a LOT of pages & collections.

So what we should do is to stop making this screen automatically in bulk:

1. We retain the checkboxes (or a Select2, see Bug 1423406), to allow for bulk operations

2. BUT we DO NOT tick any checkboxes in advance.

3. AND we DO NOT make the "current" page one of the options. You cannot untick it, because you are editing ITS permissions

4. So if you do use the bulk selector under this new paradigm, what you're saying is, "make these specific other pages have the same permissions as this current page"

This will have the advantage of 1) no longer surprising users by automatically bulk-changing permissions, 2) individualize the sharing screen, so that we can put Secret URLs onto it.