Consolidate license and README text

The license text is kept in three different formats, in four different places across three code repositories:

• Plain formatted text in voltron/LICENSE-enterprise.txt and couchdbx-app/makedmg/LICENSE.enterprise.txt

• Plain unformatted text (no newlines) in ns_server's EE-subscription_license_agreement.txt

• HTML (manually converted) in couchdbx/Couchbase Server/Credits.enterprise.html

There are parallel Community license files as well.

This is tedious and risky; we've had to delay releases solely because we didn't get all the right license text files into the right places.

Ideally we'd have one canonical location and always read it from there, and perform automated format conversions where necessary. At a bare minimum we should have all processes read the file from the same location so we can maintain it one place.

I'm filing this as an "installer" ticket because three of the four locations are installer-related.