Make new pricing tiers reflected in feature access

Created by: dadlerj

From @juliasourceress :

Just had a client call and they're interested in our middle Enterprise Plus tier. Chatted with Dan and it sounds like we don't currently gate nor get pings back on usage of any of those "Plus" features (I think most notable ones are respecting repo permissions & extensions whitelist, but dan please chime in). Could we slot either a gate or at least a ping back/stats on those into the next release? Getting this info from clients will definitely impact new ARR as well as upsell potential.

And my response:

Looks like we DO have enforcement for this (see https://sourcegraph.com/github.com/sourcegraph/sourcegraph@837b3b25d94e180864c7d1b7276a235a1ed9933a/-/blob/enterprise/cmd/frontend/internal/licensing/features.go#L19:2) but that we are not yet restricting it from basic Enterprise tier: https://sourcegraph.com/github.com/sourcegraph/sourcegraph@837b3b25d94e180864c7d1b7276a235a1ed9933a/-/blob/enterprise/cmd/frontend/internal/licensing/features.go#L45:23

So this might be an easy update actually!

Basically we just need to add the new tiers (Enterprise Plus and Elite) to our backend and update features to map to them: https://sourcegraph.com/github.com/sourcegraph/sourcegraph@837b3b2/-/blob/enterprise/cmd/frontend/internal/dotcom/billing/plans.go#L27