fix: add Time type definition for gqlgen

Gqlgen gets angry because it doesn't recognize the Time datatype. The
solution is to define a separate graphql schema with the Time type
defined before it generates the rest of our schema for us.

See 99designs/gqlgen#1463

We already had gqlgen configured to look for an archivista.graphql file
for this reason (see gqlgen.yaml), but apparently we did not check it
in.

archivista.graphql

@@ -0,0 +1 @@
+scalar Time