[bug] Error when cloning a template that already have a character count limit

[minhng99]

Hi, I'm trying to clone a template that has a pretty long title (62 characters) but if I clone it, the new cloned template will have an added prefix (Clone) to it which makes it goes over the limit of 64 characters, resulted in Django error

==> openwisp2.log <==
[ERROR 2023-07-24 03:13:47,346] module: log, process: 862367, thread: 140614550849344
Internal Server Error: /admin/config/template/
Traceback (most recent call last):
  File "/opt/openwisp2/env/lib/python3.9/site-packages/django/core/handlers/exception.py", line 56, in inner
    response = get_response(request)
  File "/opt/openwisp2/env/lib/python3.9/site-packages/django/core/handlers/base.py", line 197, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/opt/openwisp2/env/lib/python3.9/site-packages/django/contrib/admin/options.py", line 683, in wrapper
    return self.admin_site.admin_view(view)(*args, **kwargs)
  File "/opt/openwisp2/env/lib/python3.9/site-packages/django/utils/decorators.py", line 134, in _wrapped_view
    response = view_func(request, *args, **kwargs)
  File "/opt/openwisp2/env/lib/python3.9/site-packages/django/views/decorators/cache.py", line 62, in _wrapped_view_func
    response = view_func(request, *args, **kwargs)
  File "/opt/openwisp2/env/lib/python3.9/site-packages/django/contrib/admin/sites.py", line 242, in inner
    return view(request, *args, **kwargs)
  File "/opt/openwisp2/env/lib/python3.9/site-packages/reversion/admin.py", line 235, in changelist_view
    return super().changelist_view(request, context)
  File "/opt/openwisp2/env/lib/python3.9/site-packages/django/utils/decorators.py", line 46, in _wrapper
    return bound_method(*args, **kwargs)
  File "/opt/openwisp2/env/lib/python3.9/site-packages/django/utils/decorators.py", line 134, in _wrapped_view
    response = view_func(request, *args, **kwargs)
  File "/opt/openwisp2/env/lib/python3.9/site-packages/django/contrib/admin/options.py", line 1984, in changelist_view
    response = self.response_action(
  File "/opt/openwisp2/env/lib/python3.9/site-packages/django/contrib/admin/options.py", line 1589, in response_action
    response = func(self, request, queryset)
  File "/opt/openwisp2/env/lib/python3.9/site-packages/openwisp_controller/config/admin.py", line 685, in clone_selected_templates
    clone = template.clone(user)
  File "/opt/openwisp2/env/lib/python3.9/site-packages/openwisp_controller/config/base/template.py", line 208, in clone
    clone.full_clean()
  File "/opt/openwisp2/env/lib/python3.9/site-packages/django/db/models/base.py", line 1405, in full_clean
    raise ValidationError(errors)
django.core.exceptions.ValidationError: {'name': ['Ensure this value has at most 64 characters (it has 68).']}

I think we should have this exception caught and return an error message instead of spitting out 500.