feat: add dotcom option to view all subscriptions ordered by license …

Created by: dadlerj

…expiration date

Fixes https://github.com/sourcegraph/sourcegraph/issues/3017

NOT a priority for 3.4, no need to look at this until next week, just didn't want this to sit for a week after being finished yesterday...

No changelog entry needed due to this only applying on dotcom?

I don't think there's a super easy pattern for removing the decode functionality from license.go entirely, but it is worth looking into it before merging this so we don't have duplicate base64 license key decoding happening (now in both license.go in Golang AND in subscriptions_db.go in SQ).

Finally, not sure about who the backend code owner should be for subscription stuff going forward, but I assume someone on Core :)