Avoid NoReverseMatch exception during app startup if navigation has a bad link

[glennmatthews]

Fixes: #N/A

I developed this fix while investigating #785 - while it doesn't actually address that issue (see #788 for that) I think it's still useful.

If a navigation.py (core app or plugin-defined) defines nav menu items with an invalid link name (one that cannot be successfully reverse()d), as implemented Nautobot startup will fail with a NoReverseMatch exception:

Traceback (most recent call last):
  File "/usr/local/bin/nautobot-server", line 5, in <module>
    main()
  File "/source/nautobot/core/cli.py", line 62, in main
    initializer=_configure_settings,  # Called after defaults
  File "/source/nautobot/core/runner/runner.py", line 266, in run_app
    management.execute_from_command_line([runner_name, command] + command_args)
  File "/usr/local/lib/python3.6/site-packages/django/core/management/__init__.py", line 401, in execute_from_command_line
    utility.execute()
  File "/usr/local/lib/python3.6/site-packages/django/core/management/__init__.py", line 377, in execute
    django.setup()
  File "/usr/local/lib/python3.6/site-packages/django/__init__.py", line 24, in setup
    apps.populate(settings.INSTALLED_APPS)
  File "/usr/local/lib/python3.6/site-packages/django/apps/registry.py", line 122, in populate
    app_config.ready()
  File "/source/nautobot/extras/plugins/__init__.py", line 101, in ready
    register_plugin_menu_items(self.verbose_name, menu_items)
  File "/source/nautobot/extras/plugins/__init__.py", line 358, in register_plugin_menu_items
    weight=new_menu_button_weight,
  File "/source/nautobot/core/apps/__init__.py", line 286, in __init__
    reverse(link)
  File "/usr/local/lib/python3.6/site-packages/django/urls/base.py", line 87, in reverse
    return iri_to_uri(resolver._reverse_with_prefix(view, prefix, *args, **kwargs))
  File "/usr/local/lib/python3.6/site-packages/django/urls/resolvers.py", line 685, in _reverse_with_prefix
    raise NoReverseMatch(msg)
django.urls.exceptions.NoReverseMatch: Reverse for 'dummymodel_add_foo' not found. 'dummymodel_add_foo' is not a valid view function or pattern name.

Additionally it's not ideal for us to be calling reverse() during initial AppConfig loading at all, as there may be some timing or sequencing issues wherein an app is ready before all urlpatterns have been fully registered (this was my working theory on investigating #785 that led to this fix).

The solution delivered here is to remove the init-time reverse() check, and additionally handle bad URLs in the rendering of the nav-menu template, rather than failing the entire template rendering and throwing an exception to the user. If a bad URL is registered in the nav menu, the UI will now show this:

[carbonarok]

LGTM