basic unique name implementation (#123)

controller/store/sql/postgresql/015_v0_4_19_share_unique_name_constraint.sql

@@ -0,0 +1,7 @@
+-- +migrate Up
+
+-- remove the old unique index (which did not respect the deleted flag)
+ALTER TABLE shares DROP CONSTRAINT shares_token_key;
+
+-- add a new unique index which only constrains uniqueness for not-deleted rows
+CREATE UNIQUE INDEX shares_token_idx ON shares(token) WHERE deleted is false;