refactor: reference human id instead of email (#73)

* refactor: update api and client to reference humanIds

* fix: prevent deletion of network member

This may cause various side effects if there is data in other services
which reference this member

go/migrations/000008_network_invitations.up.sql

@@ -0,0 +1,6 @@
+CREATE TABLE network_invitations (
+    network_id TEXT NOT NULL REFERENCES networks(id) ON DELETE CASCADE,
+    email VARCHAR(255) NOT NULL,
+    created_at TIMESTAMPTZ DEFAULT NOW(),
+    PRIMARY KEY (network_id, email)
+);