collection nav item spills over to other journal

[Beethro]

Describe the bug
The nav items shown in the content manager aren't filtered properly. In particular collection nav items from journal A can be seen in the content manager for journal B

Janeway version
clean install from current master

To Reproduce
Steps to reproduce the behavior:

1. Install janeway with 2 journals

The table journal_issuetype contains 4 rows
id=1: issue for journal A
id=2: collection for journal A
id=3: issue for journal B
id=4: collection for journal B
The table cms_navigationitem is empty.
3. Open the content manager for journal A, toggle collection nav.
cms_navigationitem now has a single row with object_id=2
4. Open the content manager for journal B
You see the collection nav item from journal A...

Additional context
In cms/views.py there is
@editor_user_required
def nav(request, nav_id=None):
...
top_nav_items = models.NavigationItem.objects.filter(content_type=request.model_content_type,
object_id=request.site_type.pk,
top_level_nav__isnull=True)
...

object_id=request.site_pk ist wrong, for journal B the value is 2 which is the same as id for journal A's collection in journal_issuetype. For journal B we need 3 here to get the correct nav items, so maybe replace by something like
issue_type = journal_models.IssueType.objects.get(journal=request.site_type.pk, code='issue')
top_nav_items = models.NavigationItem.objects.filter(content_type=request.model_content_type,
object_id=issue_type.pk,
top_level_nav__isnull=True)

[ajrbyers]

hi @Beethro we will be totally revamping the CMS system as part of 1.3.8, I've noted this issue in #509 to ensure that the new system filters all nav items properly.