Show version number

A new script `scripts/build_release` runs `build/run dist -Dversion="[version_name]"`. Run it without arguments for a usage statement.

The `build-zip` task now maintains a file called `common/ARCHIVESSPACE_VERSION` that contains the latest build version.

The contents of the version file can be referenced by `ASConstants.VERSION`.

The version name is exposed in the following places:
-  backend endpoint /version
-  in the footer of the staff and public apps (appears at bottom right of every page)

The backend also maintains an AgentSoftware record that represents the ArchivesSpace application itself. The AgentSoftware model now ensures that that record contains the correct version.