feat: add scope support for BGP Communities (Site, Region, SiteGroup, Location, Rack, RackGroup)

[Etibru]

Fixes: #306

Implement a GenericForeignKey-based scope field on the Community model, following the pattern used in NetBox core (e.g. VLANGroup scoping). This approach is cleaner and more future-proof than adding one FK per object type.

Concretely, this means adding to the model:

scope_type   = models.ForeignKey(ContentType, null=True, blank=True, ...)
scope_id     = models.PositiveBigIntegerField(null=True, blank=True)
scope        = GenericForeignKey("scope_type", "scope_id")

with scope_type limited to the following content types:

Scope object	App / model	Typical use case
Site	dcim.site	Per-site communities (local-pref, blackhole…)
Region	dcim.region	Regional traffic engineering
SiteGroup	dcim.sitegroup	Logical grouping of sites
Location	dcim.location	Building / floor / room granularity
Rack	dcim.rack	Per-rack communities (e.g. colocation billing)
RackGroup	dcim.rackgroup	Row or cage-level grouping