Clarify use of the word license, free user, plan
Created by: malomarrec
- stop using "license" when ambiguous
- use "plan" everywhere and not "tier"
- stop using "free user" because ambiguous
- use "Sourcegraph Enterprise" and "Sourcegraph OSS" (not "Enterprise Sourcegraph")
- merge two articles about OSS vs Enterprise
- clean up the definition of plan