fixes for #10070 make l2vpn slug unique

[arthanson]

Fixes: #10070

Adds unique constraint to L2VPN slug field along with max_length. Note: Would cause issues migrating if someone already has non-unique slugs defined (probably fairly unlikely).

[jeremystretch]

Looks like a few of the tests are failing simply because they're creating L2VPNs without slug values (which are now required):

netbox/netbox/ipam/tests/test_api.py

Lines 975 to 980 in deee366

	l2vpns = (
	L2VPN(name='L2VPN 1', type='vxlan', identifier=650001),
	L2VPN(name='L2VPN 2', type='vpws', identifier=650002),
	L2VPN(name='L2VPN 3', type='vpls'), # No RD
	)
	L2VPN.objects.bulk_create(l2vpns)

[jeremystretch]

@arthanson I've just pushed a tweak to develop to add slugs for the affected tests. If you rebase this branch it should pass CI now.

[DanSheps]

Would it make sense to make slugs unique but non-required?

[jeremystretch]

Every model which includes a slug field currently requires that field to be populated. Barring any changes per #8113, let's keep consistent with that.