Use client-side routing for multiple (all?) versions

[nickpeihl]

We may be able to eliminate some tech debt by using client-side routing to handle EMS versions rather than build each EMS version in separate branches.

We could use something like react-router to check the current pathname and return the appropriate EMS manifest, e.g. https://maps.elastic.co/v7.6 automatically returns EMS 7.6 manifest.

Pros

• Makes dependency updates simpler
• Don't need to backport changes
• Requires fewer Jenkins jobs

Cons

• Need extra logic to handle breaking changes between EMS versions (ex. v2->v6.6 manifest changes, raster vs. vector tiles).
• Static file hosting providers need to redirect pathnames to root (ex. /v7.6 to /) for client-side routing to work.