Allow for versioning identifiers in URLs

[shainaraskas]

To future-proof our system, allow for some version identifier in URLs. This will keep the URLs live and pointing to the right place if we introduce versioning through URLs down the road.

This could be:

• A segment of the URL that we ignore/redirect
• A query param that we throw away

Use case: doc links from ES code

Related issue: #449

[shainaraskas]

Consideration: do we need to fix link builders anywhere else? Kibana?