Rename DomainBase -> Domain (#1725)

* Rename DomainBase -> Domain

This was a long time coming, but we couldn't do it until we left Datastore, as
the Java class name has to match the Datastore entity name.

Subsequent PRs will rename ContactResource to Contact and HostResource to Host,
so that everything matches the SQL table names (and is shorter!).

* Merge branch 'master' into rename-domainbase

docs/architecture.md

@@ -335,7 +335,7 @@ registry codebase:
     exported.
 *   `DomainApplicationIndex` -- These hold domain applications received during
     the sunrise period.
-*   `DomainBase` -- These hold the ICANN domain information.
+*   `Domain` -- These hold the ICANN domain information.
 *   `DomainRecord` -- These are used during the DNS update process.
 *   `EntityGroupRoot` -- There is only one EntityGroupRoot entity, which serves
     as the Datastore parent of many other entities.

docs/code-structure.md

@@ -142,7 +142,7 @@ encapsulate related values together.
 The classes that extend `EppResource` (along with the RFCs that define them) are
 as follows:
 
-*   `DomainBase` ([RFC 5731](https://tools.ietf.org/html/rfc5731)), further
+*   `Domain` ([RFC 5731](https://tools.ietf.org/html/rfc5731)), further
     broken down into:
     *   `DomainApplication`, an application for a domain submitted during (e.g.)
         sunrise or landrush

docs/developing.md

@@ -80,7 +80,7 @@ as follows:
 6.  Delete the old fields, their indexes, and the `@OnSave` method.
 7.  Deploy the new version of the app. The schema migration is now complete.
 
-The migration away from using a wrapper class around Keys on `DomainBase`
+The migration away from using a wrapper class around Keys on `Domain`
 objects is instructive as an example:
 
 *   [Step